The two tubes (final audio drivers) you refer to are important, the other two that are important are the two 12AX7 tubes closest to the front panel (these are the first stage driver tubes, if that's the right term). Telefunkens should work great in all 4 locations.
Fisher 500C Tubes
Which tubes (12AX7's) are most influencial to the output of CD audio through the AUX input? Is it the pair next to the phase inverter pots? I ask because I am thinking about buying a matched pair of Telefunken 12AX7's and I want to put them where they will do the most good. The receiver has 9 12AX7's all together.