[morzh]

Today's DMM will give you a true RMS reading (my Fluke 87 is good up to 200kHz) so what you could do is feed a signal and at the same time measure it with a DMM, then set the signal to peak exactly at say full screen with even number of division p-p, read the DMM and multiply it by 2.828 (2x1.414) and then divide it by the number of the divisions you have p-p, this will be your V/div.

As for the Horz - you will need a counter, then - same thing, a known frequency and qty of periods per div (make it even).

As for the input signals, it is always better to test an amp with a real audio oscillator with adjustable amplitude. You should know what your input max at the full power is, then set it, feed it in and then see at what level of volume if any you distort.