For feedback problems, try rolling the gain knob down abit. I know it seems heresy to have your settings less-than-dimed, but alot of times you can still get enough gain at 5 or 6 without the high-end hairy-ness that super high gain adds. I don't know about your other eq settings, but you might roll your highs down, just a smidge. I know alot of metal folks like the bass/highs cranked and the mids scooped out, but I actually like a big fat midrange...
but I also have a vintage sunn(O))) Model T that crushes
Otherwise:
What kind of pickups does your guitar have?
Shitty pickups are reasonably cheap and easy to replace, and they're usually a main culpriit in feedback.
Check your cables, too. Shitty cables that aren't shielded can actually mess with your signal quite a bit.
Also, I'd check your rehearsal space for possible sources of interference, like tv's, electric fans (rehearsal spots can and do get HOT AS HELL), flourescent lights...all this stuff will mess with your signal and cause feedback.
As far as getting a cheap tube amp...Fender bought the Sunn(O))) company years ago, and recently they reissued that Model T. It's got a great igh gain channel, as well as a sparkley, high-headroom clean channel if you want to run your own distortion and effects; it has a superb effects loop with separate send/receive volumes for each channel, and an auto-bias feature for the power tubes.
Fender discontinued them, and you can find them new or nearly new (with the warranty) on Ebay for $300-400. Use the rest to get yourself a second hand cabinet with Vintage 30 speakers, and you'll be crushing heads on a budget.
