I owned both the original M400, the cube-shaped amplifier in which Carver introduced his magnetic field power supply technology, and the subsequent M400t version.
The original M400 sounded quite poor. The M400t sounded surprisingly good, and was certainly an excellent value in relation to its price and its power capability (200W/channel). The "t" denoted "transfer function modification," which meant in this case that it was designed to emulate the sound of a Mark Levinson ML2, which was a pure Class A megabuck monoblock amplifier of the time. Carver demonstrated that by running both his amplifier and an ML2 off of the same input signal (music, not a test signal), electronically subtracting the output of one amplifier from the output of another, and showing that he got a reasonably good null.
The major weakness in my experience with the M400t was that the size of the image it projected seemed somewhat constricted, particularly in comparison to the tube amps I have used.
To his credit, though, creating a 200W amplifier in that small a package, and with a relatively low price, was a remarkable accomplishment. I'll add that in my experience with it, it did sound every bit as powerful and as dynamic as its rating would suggest, although I'll qualify that by saying that I was using it with fairly easy-to-drive 7 ohm 90db speakers.
Some of his later "t" version amplifiers, not in the cube form factor, were designed to emulate the sound of a Conrad Johnson tube amplifier. I have no knowledge of how they sounded.
Regards,
-- Al
