Thanks for the input, guys. I went back to the rehearsal room yesterday with the firm purpose of making the goddamn thing work, and to a certain extent I think I accomplished my purpose.
I tried different settings in the amp, I swapped the different pedals in my chain, experimented with placements and combinations until I found the most reasonably satisfying sound (but at 11:00 pm it was like... "that's it for today man, gonna sleep now").
First I got to a very nice balance of Master Volume + Gain + Output Volume; after that, I plugged my pedals to the effects loop and the first one to earn it's place was the ISP Noise Suppresor :lol: , it was a no-brainer. After I got to a point of loud volume, crushing gain and perfect noise gate setting I moved to the BBE Sonic Stomp. Another instant hit!!! The sound improved a lot, with more gain, more sparkle and more definition.
The Eq. pedal was like... yuck!!! so it's gone now; the delay sounded cool so it stayed... and so on. Now that I see it it was the Eq. pedal the source of all that annoying feedback, it ruined my sound until I removed it from the chain.
For now, I'm satisifed but maybe I'll change tubes in a month or so, just to be sure, since this is a second-hand amp and from what I understood, the former owner never replaced the tubes... :shock: . Maybe a high-gain kit from JJ Eurotubes, thought I have not decided if with 6L6s or EL34s, we'll see.