Besides country of assembly and production issues, the electronic differences are:
1) 800B & C can bridge 4 ohms, 800A can bridge 8. The B & C have the 6 output transistor power amp and beefed up power supply; the A has the older 4 output transistor design.
2) Unmodified 800As are a little noisier than the later or modded models. The noise is independent of the master gain setting.
3) The B & C have a fan change that lets it move more air.
A friend and fellow bassist was over today to do a gear mix 'n match. We had my Edens (800A and 405), a loaner oldie SWR 400, and his G-K 1001RB, and an Aggie 1x12, an Avatar 2x10, and 2 Warwick 2x10s. Did some tones, white noise, iPod, and bass playing through the gear in various combinations. We ended up ranking the amps by warmth thus:
1) WT800A (could almost be described as "vintage" tone)
2) G-K 1001RB
3) WT405
4) SWR400
The SWR400 was by far the flattest, cleanest sounding with all the knobs set flat though. They are all great amps, not an average one in the bunch. Best part is, you can find great deals on all of them.