First Acoustic Research products included AR-XA turntable, AR-5 and AR6 speakers with Dynaco amplification and Quadradaptor back in the 70's. My best system ever until last year, when I discovered the AR Holographic Imaging Series speakers.
Started with M3's, graduated to M4's, then M5's with MC1 center channel speaker in HTS setup.
Typical AR sound: extended and flat frequency response, low distortion, broad dispersion; early holographic!
Finally got set of M6's month ago. Synergistic addition of extra 8" bandpass woofer (vs. M5 or M4.5) produced a sound and range comparable with any affordable speaker I have ever heard. Muscular yet clean is the way I'd describe them. Less of the sometimes clinical sound characteristic of AR speakers in general.
Just as comfortable with large scale symphonic, big band and full-on rock and roll music of any style.
In combination with AR Hi-Def series Front AR RC-2 and Rear RC-4 center channel speakers hooked up to 6.1 channel Marantz SR8200 AVR, comparable with just about system I've ever heard!

I have had these speakers since 1992, when I bought them as new. Just recently I thought that I am ready to invest in new set of "really good speakers", but after listening several new ones ranging between $800 - 1500, I could not see the point. Even that these are old, they still perform really well.

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Superb stereo imaging with great midrange and crystal clear airy highs. Well engineered and stylish design with the ability to handle a good deal of power. Small footprint allows many opportunities for placement. Surprisingly good in home theater use also.

The midrange and high end can be a bit harsh at loud listening levels if not toned down. The bass response is quite effective considering the size of the speakers , but these are no bass powerhouses.

The Holographic Imaging Series were designed to provide vivid recreation of the sonic space on a recording and provide the exact positioning of instruments and singers in that space. The M 4.5 model is difficult to find information on and is apparently very rare (serial numbers on model pictured number in the 3000's). These speakers have amazing presence and holographic imaging characteristics with particularly clear and crisp midrange and high end compliance. These speakers also have bi-amping capability.
The 8" woofer is a thick pulp paper cone that fires upward into the sealed lower speaker chamber. This chamber is separate from the upper chamber that houses the midrange and tweeter and is ported at the bottom of the speaker and on both sides also. The side ports are covered with removable black foam inserts. This offers the control of an acoustic suspension speaker and also the sensitivity of the vented side. The bass response is good and what I consider adequate for a speaker this size (36"x8.5"x10.5") The small footprint and stylish looks are a plus for many listening environments. The bass response is enhanced by locating the speakers near a wall or in corner. The technology behind the bass woofer design is neat but the real excitement is in the upper chamber.
The 8" midrange with high excursion rubber roll surrounds offers superb stereo imaging and helps provide a realistic sound stage. A few times I have been startled by the speakers sound while they were playing a movie and thought the sound had come from another part of the house rather than from the speakers. The midrange speakers are angled upward about 30 degrees to provide a smooth flat sound.
The ultra wide dispersion tweeter produces crystal clear highs throughout the room and contribute much to the musical detail. These speakers reproduce music in a detailed manner that I have enjoyed through headphones but yet the sound is much more expansive and realistic due to their holographic imaging properties.The M 4.5s require a rather powerful amp to adequately drive them. At the time of this review I am running them with a Carver Sonic Holography amp rated at 100 watts per channel. It is a great compatible combination but one that can be a bit much on some recordings ( I simply turn the Holographic imaging off). To my surprise the M 4.5s make excellent home theater speakers also and have a kind of surround sound effect with movies.The holographic effect also seems to be more prominent on some recordings than others. Generally the more complex the recording the more pronounced the effect, especially with digital recordings(ex New age, jazz, classical). While the M4.5s will play quite loud without distortion they are not powerhouses, especially in regard to loud bass. They would do very in an apartment or small room or office and actually do quite well in a larger area. I run them concurrently with another rare pair of speakers; a pair of American Acoustics DS1212's. These have dual 12"woofers each with response down to 23 Hz and rated at over 200 watts. The M4.5's have no problem keeping up, providing the great stereo imaging and the midrange and high notes while the American Acoustics pump out the bass. Either pair can be switched on or off to tailor the particular musical listening experience.

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I have listened to the Bose 501 V model many times and can say that the M 4.5 is a better sounding and better imaging speaker. Better design, bigger woofer and bigger midrange. This speaker is also quite solid at 35 pounds. There were different models of Holographic Imaging speakers Acoustic Research produced both before and after the M 4.5 model. Information on these can be found at the Classic speakers page at http://classicspeakerpages.net/ar/ar.htm