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To some large extent based upon positive things said about the silverface Musicmaster Bass I had read on this fine forum, I recently got one. 1971, 2 GE 6aq5a power tubes, unknown 12ax7 preamp tube. The only modifications was the "Harvard" mod which apparently fixed the tone control, though I'm not sure how. The tone knob does seem to be functional; I've heard that normally you just have to crank it. The speaker was uprgaded to a Weber 30W 12A125A, which is supposed to be a tweedy sort of vintage thing.

I like the amp very much. I would like to try to get a little more clean headroom on it (gosh seems like people are always posting about that--though I searched and there didn't seem to be any threads about this amp dedicated to that topic). I'm thinking about swapping out the preamp tube for a 12at7 or 12ay7. Any tips on that account? I'm also wondering if changing the speaker would make a difference.

I really love the amp and how it sounds. But it starts breaking up at around 3 or 4 with a normal guitar. With my LP's weird hot HB-L and HB-R humbuckers, it is already quite broken up at 3. I'd like to try to get the amp to stay clean at a slightly higher volume.

Incidentally, the amp "takes pedals well"--extremely well. From octave fuzz to whatever. Also, when it is cranked to about 8 or higher, it ends up with awesomely ugly fuzz tone that I dig but my neighbors probably don't.

I had a MMB amp and it was lovely. I traded it for a guitar, but I definitely would like another one. They are like a mini Bassman. I hope Haze still has my old one.

Well I got some preamp tubes to swap for fun, a Sovtek 5751 and an EHX 12ay7.

The EHX 12ay7 gives me clean forever, especially with single coils, all the way up the volume dial. The 5751 gets crunchier earlier, a little later on the dial than the unknown brand 12ax7 that came with the amp. But I think the 5751 has a little more harmonic complexity to it as it is cranked. Very cool.

Posting this so that if someone else has a similar question, there may be some answers for him/her!