I have a 1959 Gibson GA 8 1-12. When I turn the volume past 5 there is a noticeable volume drop, and no change in volume from 5 to 9. When I put it to 10 it's real fuzz box type sounding. Any idea's on what the problem might be?
Sounds OK to me. I've owned a bunch of '50s Gibsons and they are just as you describe. I wouldn't call it "fuzz", but it is pretty heavy overdrive at 10. Some of my older Marshalls have have all their volume at 3, and from their up it just gets greasier. Lower output pickups will give you a bit more range before clipping.
We're talking tube overdrive here, I don't really see a need to get technical.
My volume tops out at about 4 or 5 (depending on the channel). After that it's overdriving the power tubes harder, but not really getting any louder, at 10 it's distorting the hell out of my power section.
If it's loud cleans you want, you'll need an amp with more clean headroom (the amount of clean volume you can achieve before tube breakup). The nice thing about 50w amps is that they break up at lower volumes. A 100w amp will give you a ton more clean headroom. You could also try using a lower output pickup, rolling off the guitar's volume, installing a pair of KT88 power tubes (if there's room), or micing the amp and listening through a loud monitor or in-ear monitor.