I just got my backup tube head from the repair shop (via UPS... repair guy is an hour away). When I play my bass through this head, it sounds like there is an auto-wah or envelope filter on the bass... there's a ping overtone on the attack, and a MWAH sound on certain notes (A and D strings mostly). The louder I turn up, the more apparent it is. When I'm playing loud, the mwah sound is very strong. IS IT AT ALL POSSIBLE (sorry for the caps) that the issue is my speaker cab, (or pickups, wiring) and not the head? Is there any situation where a speaker cab could sound like that, not caused by a bad tube? I'm asking because my repair shop comes highly recommended. It would be stunning to me if they shipped my head back, and it still has a bad tube or some other issue. I recently replaced a speaker in my cab (Ampeg 8x10) and the store clerk told me to be sure I got all 4 wires hooked up correctly. There were only two wires on the old speaker, and I assumed that was because the speaker was the last in the series of 4. O O O X <--- this one O O O O Also, I am not isolating the overtones coming from any one speaker. I can't test another head through the speaker cab... the shop still has my #1 head on their bench. When I use this head as a guitar amp through a 4x12, it sounds fine. I only hear the problem through my 8x10, when playing bass. Please tell me I didn't pay 200 dollars and drive an hour to see a 'recommended' tech, and they did a half-assed tuneup on my tube amp.... what else could be the cause, of what seems to sound like a typical bad preamp tube? Any other possible scenarios where I could be getting this sound?