I owned the M150 and 99 Magnum combo for several years. Then later on I owned the Cronus (modified by Mark O). I now am using a Perseus with a McIntosh Mc275 MkV. I think that the Perseus-McIntosh combination is the best of the bunch.
The Cronus was really good, especially in the midrange. I would characterize it as sweet sounding and very 3 dimensional especially on female vocals and acoustic music. It did great driving my Quads and my Virgos in my small room. But when I moved into a big open room, it ran out of steam.
The Perseus-McIntosh combination is extremely dynamic and transparent with better overall balance of bass, mids and treble. While it is not as sweet and dimensional, overall I find it more enjoyable and it has much more drive capabilities. Personally I think the Perseus is a more transparent and dynamic preamp than the 99 magnum, which always sounded a little veiled and prone to some upper midrange tube glare.
I can't speak to the Stereo 90, but the M150's were very good amps. I would second the advice to give Mark OBrien a call. He and Mark Walker are both great guys and very generous with their time and advice.