i just bought a gk 800rb and i was curious if it would sound good with an avatar 2x10. the head is 400 watts @4ohms and has a biamp switch (highs:100watts lows:300watts)
Sure, it'll sound fine. Just use the Low output in full range mode and you should be able to get about 200 watts out of it (assuming that your 2x10 is, in fact, an 8-ohm cabinet).
Just so you know, the 800RB runs 300w @ 4Ohms, in full range. In biamped, it'll run 300w @ 4Ohms to the lows and 100w @ 8Ohms to the highs, which would be 400w. You need 2 cabs for this though.