scoping a mono-block amp

Don't think I would want to increase the a/c input power which feeds two transformers with output to three separate power supplies: a balanced 150/150 dc supply for the front end, a 6.3 vac tube heater, and a 550 vdc B+. I'd like to see the amp without seeing the pre.

I'm wondering if an iphone might be useful; mine has a waveform app.
 
Are you trying to improve the sound by not using a preamp? That's easy and will work well,
just use a 10k volume control in a box (instead of the preamp). Make sure that your source
can drive a 10k load, but most can. If you just want to buy one, they are usually called
a passive preamp.