The amp used to listen to the difference signal is not important.  It won't have a significant effect, or at least, any effect it has is not important and won't significantly alter what you hear.  An amp that varies by +/-10dB in its resposne over the audio range would have to be catastrophically bad.

Once you get to a difference signal that is very small then the S/N ratio of the "amplifying" amp will be an issue but at that point you aren't able to make accurate judgements by ear anyway so you have to resort to using a scope to monitor the difference signal.

Michael.