Summary:
I wanted to add this review as an addition to my previous review below. As mentioned, it took five and one-half weeks of phone calls and e-mails to the Parts Connection to get my lost and damaged parts replaced. Thankfully, care had been taken to pack the replacement parts well.
The four layer PC board used in the DAC 2.6 works very well to conduct heat away from the solder joint; this makes it very easy to damage a part by overheating it. The Signature upgrade is worth the cost, but don't try to install it yourself unless you are an experienced solderer. I am an audio technician by trade, and I still found the board troublesome to work on.
Oddly, I think that what I thought was the worst part of the upgrade process, wrestling the DAC chip's filter caps out and replacing them with fancy Sanyo caps, could have been left out of the upgrade. I think it would have been just as effective to bypass the existing filter capacitors with some panasonic P-type polypropylene caps, and would definitely save the upgrader 30 minutes of frustration.
Definely try the OPA 627A op amps, they are sweet! These are easily installed as the four existing op amps are in sockets.
The Linear Technology Voltage Regulators are definitely worth the effort to install. These are the LT1033 and LT 1085 and you need one of each for both channels. The parts Connection doesn't supply the insulating mounting washers neccesary to avoid shorting out the power supply when changing to the new regulators.
What I felt was a major omission was that no Hexfred rectifiers were included in the upgrade kit even though there are provisions for them on the circuit board. These superior power supply devices are present in the more expensive DAC 3.0. Digikey.com sells International rectifier
Hexfred diodes as part number HFA08TB60-ND.
I think that changing out the semiconductors mentioned above would give you much of the Signature upgrade sound at about half the price. All of the parts are available from Digi-key.
The main reason I installed the upgrade kit was that the original DAC 2.6 tended to sound harsh and raspy on sibilants and on distorted rock recordings( what rock album doesn't have some distortion on it?). I felt the problem had to be in the DAC as my lost but not forgotten, heavily modded CDB-650, with its video op amps, did not respond as harshly. Sure enough, the upgraded unit, with its faster op amps seems much less upset by distortion in the recording. You can tell the distortion is still there, but the dac isn't overreacting to it or being driven into distortion itself by it.
There is even more detail and nuance on well recorded discs, and the poorly recorded ones aren't nearly as irritating. There is more depth and ambience to the recording. It almost sounds like someone snuck a tube into the dac .
I think that the Signature upgrade kit is the way the DAC 2.6 unit should have sounded all along. The Parts Connection should ditch the relatively harsh-sounding standard kit and then sell the Signature grade DAC unit for
the standard price. Now that would be a value!
As it stands, The Marantz/ Signature DAC2.6 combo betters any CD player I've heard for less than $2000. It is clearly superior to the industry-standard Rega Planet.
It is sweet, fast, detailed, yet it has a smooth welll-damped treble that does not fatigue the ears.
I can reccommend the DAC unit, it is sonically excellent, but I think that Sonic Frontiers/The Parts Connection has much to learn about providing good customer service. I should know, as a technician for a high-end manufacturer I provide daily technical support for our customers.

Strengths:
Much smoother, more refined sound with Signature upgrade kit. More defined soundstage and ambience.
Less upset by sibilants and distortion on recording.

Weaknesses:
Installing the upgrade kit is not recommended for inexperienced solderers. It is extremely difficult to solder and unsolder parts on the four layer PC board.
It took me five and one-half weeks to finally get all the parts from the slow and indifferent people at Parts Connection so I could install the upgrade kit.

Similar Products Used:
Marantz CD-63SE
Pooge 4 mod one Magnavox CDB-650
Assemblage Ultrajitterbug

Summary:
I must state that I bought the DAC 2.6 kit, not the 2.5 which is no longer made. The 2.6 has the 24 bit Burr Brown DAC chips which are the Sign-Magnitude type. I think this type of dac chip sounds more dynamic than the Delta-Sigma 1 bit types.
I am using the DAC unit with a Marantz CD-63SE. The Marantz is a nice unit but rather soft and unexiteing. The Parts Connection piece definitely perked things up. It is obviously very fast and detailed, conveying much more information than the Marantz could. The difference is similar to hearing a good digital remaster of a favorite old recording. The drum kit has more impact and presence and many formerly-subliminal details are clearly audible.
This impression of speed and detail was enhanced with the addition of an Ultrajitterbug jitter reduction unit.
My main complaint with the unit is that it seems to deal harshly with imperfect recordings. I would almost say that if there is distortion on the recording, it seems to upset the DAC 2.6 in a way that I never heard with my old Magnavox with its video op amps. Sibilants on poorly recorded vocals are the worst.
In an effort to reduce this effect, I ordered the Signature parts upgrade kit from the Parts Connection. The kit consists of a small handful of premium parts for $150. The kit arrived with the packets unsealed and parts missing and rattling around in the box. To make a long story short, it's three weeks later and I still have not received replacements for the missing and damaged parts. I will never deal with these people again. Their products are nice but their customer service sucks.

Strengths:
Fast and detailed, good soundstage.

Weaknesses:
Can sound harsh and unforgiving on less than perfect recordings. Poor customer service.

Similar Products Used:
Magnavox CDB-650 with Pooge4 mods
Marantz CD63SE

Summary:
I have been guilty of writing my reviews too quickly before the equipment has been sufficiently "burned in" and sounding its best.
After 100 hours burn in the upgraded unit sounded fantastic. The sound blossomed and bloomed. The direct wiring of the Kimber Silver streaks to the circuit board was a failure - the sound deteriorated noticeably so I reinstalled the rhodium plated output sockets.

Next I replaced the Burr-Brown OPA134 op amps in the final output with OPA 627 BP's. They are not cheap at $20 each. The sound was immediately clearer as if a veil had been removed. Note, more than 50 hours of burn in was required to get rid of the harshness. The overal flavor or style of the sound was unchanged.

Then I replaced the AD811 IV converters with another pair of OPA 627's. The change in sound was even more dramatic. The sound stage broadened and sharpened. Even more detail than before. Some listeners may feel the sound is too sharp and edgy but there is no denying that the detail is incredible. The already very good bass became even better. (note 100 hours of burn in were required to get the unit to sound its best)

The unit is now close to $ 900 in cost but the sound is very very good. The same mods have been carried out by another DAC 2.5 owner and we agree on the improvements which have occurred.

This unit is a must buy for tinkerers and tweakers !

Summary:
The upgrade parts finally arrived. They include better volatage regulators for the power supply, better capacitors on the outputs from the PCM 1704 DACs, better capacitors and resistors in the analog output stages, different current voltage converters, some expensive Kimber rhodium phono sockets and a piece elastomeric damping material for the cover. The installation took me about 3 hours. Only people with kit building experience should attempt this. Those with out experience should have TPC do the work. The upgrade is much more difficult than the assembly of the basic unit.
The sound is detectably improved. In my initial review, I commented that the imaging was not great and attributed it to the rest of my equipment. The imaging is now much sharper, the music seems more focused. It is hard to say much more as the unit was very good to start off with !

My next modification will be remove out the output sockets and then direct wire Kimber Silver streak cable to the printed circuit board. I will then be able to retire the audioquest turquoise interconnects

Summary:
Update on my previous review. The unit has burned in for 200 hours. The graininess I reported earlier has completely disappeared and the sound is now very sweet. I purchased 2 HDCD discs, Reference recordings' Stravinsky and the latest Jewel CD. They both sound very good, detailed, delicate in the quite passages, and thunderous in the loud passages. I am curious to know how this unit compares with the MSB Link. Any MSB owner near NYC so we can do an A/B comparison ? The problem I now have is I want to upgrade my downstream components !