On one leg or two legs?

I have two. One for power amp(s). The other for pre and source. The idea is to create "quiet" circuits that are receiving as little influence as possible from other power users in the house. Makes sense to me that amps may draw more power and therefore may create "noise" of their own. I suspect the preamp would be highly sensitive to noise so that's why I have 2. I further suppose one for each piece would be the ultimate but I don't have room in my panel for 6 (amps, pre, CD, phono pre, turntable) dedicate 20 amp circuits.

Generally, you should install them on one leg. The reason is that both legs in the panel may not be the same voltage. One could be, say, 120V and the other 118V. If you put one circuit on each, then your equipment will have a 2V potential between them when plugged in. This could cause a ground loop through the interconnects, the equipment, the power cord and the wiring back to the panel. If you place the two circuits on one leg (matters not which) then the potential difference will always be zero. However, the length of both circuit wires to the breakers should be the same because a difference in resistance resulting from unequal lengths (ohms per foot) also causes a potental difference.

I would suggest running isolated ground circuits. Each circuit has it's own ground run back to the panel to a separate ground bar. Then the Isolated ground bar has it's own ground run to it's own ground rod. If you are running two circuits, I think it would be worth the small added expense. I am not an electrician, but I play one on tv... nah, but my friend is, and we just did this, and I don't think it would matter if they are on different legs as the voltage is determined from the service wires. but if you have room on the panel, just use the same leg? If you have a meter you can check the voltage.

Why not try it on the same leg and then different legs and report back what you hear?
IMO; I agree with Gs5556 and he is a professional.