the 400RB I and II had 125 watts at 8 ohms and 200 watts at 4 ohms, the later versions had different cosmetics and specs as mentioned above.
The 800RB has two amps built in and was biamp-able. 200 at 8/300 at 4 for the "low range" amp, and 100 at 8 only for the "high range". Could run these at full range as well which is what most people seem to do.
Both are excellent amps, the classic GK sound. I have both and use them constantly.
JPJ did use an old GMT amp in the 70s on occasion. GMT then changed it's name to GK.
California Bassist #24, TB Cellist #8, Honorary Georgia Bassist