The Microtube (as Dante says) are a hybrid amp with a tube front end and ss power stage. Also the B200s is ~124 wRMS per channel into 4 ohms, so a bit more power than the MT200.
I've also got a real soft spot for the B200s, Dante you'll be (sort of) happy to know that your old one is now living at the south coast of NSW Australia with a friend of mine and now sports a nice new toroidal power transformer for Aust voltage.
My B200s developed a horrible intermittent (loud) crackle in one channel. The tech said it wasn't worth me paying him to change every component on the board to fix it as he couldn't find what was causing it (seems capacitor related to me). Anyway, I needed a power amp so bought a Carvin TS100 all valve 50watts a side. Very nice amp and I'm quite happy with it but I also miss my B200s which is now sitting in a box doing nothing. I also have the US transformer from Dante's old B200s sitting there doing nothing. Dante, maybe we can work something out here ?
The B500 was for guitar or bass I seem to remember, I've only ever seen one in Canberra, it was in a guitar rig with an MP1 I think. They're such clean transparent (and warm) amps you could use them for either