Well. Could be everything said so far. Ususally if any pickup is microphonic they're microphonic anywhere in any amp. Do a following "excluding causes" tests, and experiments:
You have turned down the volume knobs on your bass first, have you? All the way off. And even removing the cable from your bass. It shouldn't hum if you hold it up in the air. To test if the drums still puts through? If then, it's not anything on the bass.
1. Change cables, swap them around. Does it still gets heard as much?
2. Put your bass to your mouth and scream and yell into the pups! Is it heard in any amp?
3. If another guitar amp that is solid state, or PA is available test the split signal into these. I e the high treble content of the micro pog and blues driver should be able to be heard there too. If it's still, then it's not the tube amp.
4. Cables run together with tube amps (especially long cables) can turn into some kind of antenna, and together produce som kind of weird pickupsignal as well as microphonics. So : are the drums miked up in any way too? Leakage can occur anywhere down the line.
5. If your bass is active, or run in active mode, or anything else down the chain (pedals that runs on batteries) check if they (it) perform better when run on wall warts power adaptors or swap to fresh batteries.
6. Earth/ground connection, of both bass amp and gtr amp. Check that the power cables runs in the same "ground" so to speak.
It should be heard in the bass amp. Main suspect is the Fender Twin, tubes gone microphonic together with added "dodgy" things that all adds up in the end.
7. Tap with a lead pencil on ALL of the tubes at the back of the Fender Twin reverb while it's running. Be careful. Just tap lightly. If a pinging pitch note is heard, then it's them. Remedy: switch out tubes to all brand new fresh ones. You can yell into those too, but it's too weak signal there, and a little bit...unwieldy.
EDIT: Yes, the reverb "can", things. That's why I love digital reverb pedals that mimicks spring reverbs. I'd rather get rid of the noise and hum and picking up signals, and have a decent digital impersonation, than down to the bone hardcore exact spring. I've used several amps with FX/send returns and even physically removed the reverb can. Makes for a lighter amp, and saves up one tube to cause problem the 12au7. One can remove that one and use as a spare. but hey, back to topic...
