The low and high are for the same tubes. It just reduces the output level from 130 WRMS to about 65 WRMS. Not much use for bass, but was supposed to allow guitarists to run the power amp into distortion for the sound that only a sagging tube power amp can deliver. In my opinion the problem with that is that you also were running the solid state pre into distortion at that point...
As for bass, my HD-130 (had the 12AX7 driver, bought new in December 1977) wasn't particularly noisy at all. The bright switch does add a lot of high end, and if you're running the pre hot it will also amplify any noise in the circuit. Good 6CA7 power tubes are important, and set the bias according to the schematic. I sold my HD-130 about ten years ago so I don't recall, but it was a simple (assuming you know your way around the insides of high voltage tube amps and you KNOW safety precautions- if you don't know this stuff, then take it to a GOOD tube amp person) process of reading the voltage between one pin of a power tube and ground, and setting the voltage to the exact spec listed in the schematic that came with my amp.
Make sure you have the impedance switch set to the correct impedance for the load you're running. And note that the speaker jacks on a Music Man HD-130 (and a 65) are wired in SERIES, not the more common parallel. That only matters if you're using two cabinets. Hooking two 8 ohm cabinets to the back of the amp gives you a 16 ohm load, not a 4 ohm load. If you want to use two 8 ohm boxes, daisy chain them together because every speaker cabinet I've seen in 30+ years has parallel connections. This feature on the MM amps gave you a lot of flexibility. If you have two 4 ohm cabs, hooking both to the amp gave you an 8 ohm load. Two 8 ohm boxes could be wired daisy-chain to get a 4 ohm load, or you could use four 8 ohm cabinets by hooking pairs of them together in parallel, and then running each pair into the back of the amp...
Also note that the "Bass" channel was designed for "bass heavy" instruments and it might sound better to run bass through the Normal channel. Try it both ways.
The HD-130 WITHOUT reverb was the bass head. No reverb, the Deep switch, and a bass channel with the EQ centers chosen for bass. I believe the Normal channel on both the HD-130 and the HD-130 Reverb were the same circuits. The guitar amps had reverb and tremelo and the "Vibrato" channel had the same EQ centers that the Normal channel has.
As for how useful they are for bass, it's pretty subjective. I liked mine, and would really like to have it back. But that might just be nostalgia too. I was a Fender dealer, but the Bassman 100/135 was not half the bass amp the Music Man was. That's why I bought a MM HD-130 even though I was a Fender dealer. More robust power amp that held together more cleanly, better physical construction, bigger transformers, and sounded a lot better to me.
BTW, I know the post is five years old, but I hope bassgreaser retubed his amp with the right tubes. They were NOT designed for 6L6 tubes at all and the circuits were in no way related to the Fender Bassman 135 amps. The only thing in common besides Leo Fender's original design (which isn't all that original) of the Bassman was the basic cosmetics- two channels, one for labeled bass, one labeled normal, and the basic shape.
Also, the OP talks about his power tubes being broken "probably by shoving the footswitch in". There's no footswitch with an HD-130, and nothing that could be switched. If that amp has a footswitch and footswitch jacks, it's the HD-130 Reverb guitar head.
John
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