The OP appears to just want to measure headroom and simple signal stuff, not harmonics down to -120dB, so perhaps any form of battery powered linear amp with sufficient output signal level would seem fine - including an opamp, assuming it can drive the anticipated lowest loads at the max signal. May need to use a few 12V batteries, or 9V batteries, and an opamp with say 30V or +/-15V supply, or multiple of batteries. I have an LM1875 board for that purpose, with a suitable gain to just not clip with available level from my soundcard, but there would be many other simple boards on ebay etc.