Up until yesterday all my testing was done with 4AA batteries for power, producing about 5.6V. Then I finally picked up some 9V for my smoke alarms and decided to try one of them in the circuit. Plain 'ol Duracell, measured at 9.6V. Holy poop, talk about audible differences! Everything just became that much cleaner and defined. I swear it was like listening to a different headphone, no exaggeration.
Of note, using two 9V batteries did not make a difference (at least not a very noticeable one). I guess when I was using just the 4AA it was running near the lower limit of the opamp which did not let the opamp perform optimally.
edit:
opamp RC4560, voltage range +/- 2~16
with the 4AA, I had +/- 2.8 (a bit under actually)
with the 9V, I had +/- 4.8
I suppose current draw might be a factor too, but I don't know what the batteries are capable of supplying, and I don't think it would be a limiting factor here anyways (please correct me if I'm wrong).