An oscilloscope (or even an "audio probe" connected to the instrument input of an audio interface, plus RoomEQ Wizard or a scope plugin in your DAW) would be really helpful too...
Well, "flying blind" isn't exactly very constructive either...
Do you have a piece of solid-core copper wire, a 100nF 100-250V film cap, a 10k and a 1meg resistor, some length of audio cable (one core plus shield) and a 1/4" male jack, some heatshrink and some basic soldering skills? I assume you at least have an audio interface..?