Having owned the Servo 15 v1 for over 5 years I was used to great bass. Looking at the numbers (400 vs 1700 RMS) you might think like I did that it would be a massive difference. It is not. The new Sub 15 looks better than the older box unit. It does play louder, cleaner and lower than the Servo. I have yet to push it to distortions levels and it can certainly do things that the Servo v1 could not. However in reality it is only about a 15-20% improvement. The price I initially paid for my Servo v1 versus the Sub 15 has naturally increased over time but I still feel if you can get a Servo v1 or v2 for a good price grab that (as long as you only plan on running one sub). The Sub 15 has one MAJOR plus in that it supports the PDK1. Using PC software and a MIC you are able to equalize the SUB to the acoustics in your room. This also is great if you have more than one sub. It can have a major impact on the quality of bass but will not completely overcome poor placement and poor room acoustics. It has built in phase and crossover dials unlike the Servo that required the external X30.
In the end you buy this sub for really clean deep bass that you can hear and FEEL.

All things considered... it is a fantastic subwoofer just like it predecessors, just not sure on the price premium.