My USB interface is limited in the maximum input level it can take before clipping. It works fine for microphone input though. If I were going to analyze power amp outputs, I'd need an interface with gain controls and probably an attenuator before the input to avoid overload.

I use the 1/24th octave resolution in TrueRTA all the time. You can still get a good idea about what your system is doing with 1/6th octave resolution, but you might miss some really sharp peaks and dips. The white noise feature is basically useless. Pink noise is the only source that needs to be used for audio (aside from discrete sine other waveforms).

However as they say "....as long as I'm spending that much, why not get everything?"