Hi oddball;
Bias: The 8417 TBass is a 'fixed-', not cathode-, biased amp and has three trim pots; one marked 'hum balance' that balances the AC on the tube heaters. The other two trim pots are on the chassis deck adjacent to the output tube sockets. If you have a meter, set it to DC voltage, connect the black lead to the chassis, turn the amp on with a speaker connected to the output transformer, and, for the left output tube (as viewed from the rear), put the red lead into the left hole in the test point fitting.
You are looking for .52vdc on the meter; if you don't get it, bump the nearest-to-the-tube left pot back and forth until the meter reads .52vdc. If you don't get it spot on, leave it and try the other tube; same sequence, right hole on the test fitting, move the pot until you get the corrrect reading. If you get it on one and not the other, either the tube is too far gone or either the 1% precision 6.8ohm cathode bias, 2.7K, or 91K grid bias supply have drifted out of tolerance. If you know the tube is good, clean both pots with spray cleaner and do over. If still no good then one or more of either the bias network or bias supply resistors are out of tolerance and a technical intervention is necesary. Same process for the 6L6 except .41vdc bias voltage although I'm not sure the 8417 bias supply can reach that point; mine didn't.
This thread for 8417/6L6 power and bias discussion
Discussion of TBass 8417/6L6 Amps
This thread for locating bias pots and test point on the 8417 TBass
Includes pics of pots and test point
Unlike cathode-biased amps, the bias pots allow the use of mis-matched tubes; they don't age evenly but the pots/test point allow you to keep the bias on them the same.
Pre-amp tubes: The 12AX7 has a higher gain factor than the 12AY7; you may get cleaner sound with the 12AY7 but it will be at the expense of some gain. They are plug-and-play substitutes otherwise. It's just a suggestion but you ought to try both and leave the one that gives the most pleasing result.
cj