Reddit mentions of DSO Nano V3 Pocket-Size 32-Bit Digital Oscilloscope Kit USB Rechargeable 1Msps 0-200KHz 10mV/div with 2.8" Color TFT LCD Waveform Storage and Playback 6 Triggering Modes Built-in Signal Generator

Are you in college? Or is there a nearby college you can make friends with? This is the sort of equipment they should absolutely have on hand.

If you're able to transport yourself, this may be a great place to look: https://www.electronicsfleamarket.com/schedule

Otherwise, I've no experience with these, but you might check out standalone units like this: https://www.amazon.com/Handheld-Oscilloscopes-DSO-Nano-V3/dp/B015X6LZFO

There's also stuff like this where you might be able to get more bang for your buck if you're able to supply a computer: https://www.electronicproducts.com/Test_and_Measurement/Benchtop_Rack_Mountable/Top_7_PC_based_USB_oscilloscopes_of_2017_for_hobbyists_makers_and_pros.aspx

Or go the engineering route and DIY: https://hackaday.com/tag/diy-oscilloscope/

I use an O'tool module for a scope which I like because it's a lot smaller than the Mordax. You can probably score a used version 1 pretty easily.

There's also this:
https://www.amazon.com/Handheld-Oscilloscopes-DSO-Nano-V3/dp/B015X6LZFO
There's a cheaper version of that somewhere as well.

EDIT: Found it. https://www.adafruit.com/product/468

This one is the newest version. Needs a firmware update, but once you get update, it works great.

You might ask /r/audiorepair to see if anyone has a lead on a schematic. Make sure the output relay is clicking and try using the front/rear channels as well. In cases like this it helps to have an oscilliscope to probe the signal as it goes through the amp and see where it stops. I have a $85 DSO Nano V3 specifically for this and the fact that it's battery powered helps keep things safe since there is no path to ground.

Cool, those ESS have Heil AMTs. Sounds like a good project.