how to use a spectrum analyzer?
A voltage divider, assuming the amp has low output impedance and the analyzer has high input impedance, should affect all frequencies equally, so you don't care all that much.  In fact, since log(AB)=log(A)+log(B), voltage division just means you are subtracting a constant from all dB readings.  What frequency range are you interested in, by the way?  Sounds like you're interested in audio, but I just want to make sure we're not talking about very high frequencies, where non-ideal resistor-type things might have a real impact.