Output test on amp

An inexpensive signal generator is what you need.

Connect the signal generator to the amp, with level all the way down.

Get a DB level meter and from the lowest frequency to the highest, go for it.

Set it at 1KHZ first and a decent output level from your amp as measured on the DB meter.

Then go to 10 HZ, 20 HZ, etc. up to 20 KHZ and see if the output stays flat.

It should rise up to the -3DB point and then stay relatively flat until around 20 KHZ and start to roll off.

enjoy

Also, you want to measure this directly from the amp, not through the pre-amp to the amp.

in addition, if you really want this done right.  use a Osciloscope instead of a DB meter.

But, we are talking inexpensive here.  So a Radio Shack DB meter (or app on cell phone), and an inexpensive signal generator.  

Plug the output of the signal generator into one channel of the amp, Set the DB meter 1 meter away from the speaker and start the test.

enjoy

I believe erik’s suggestion using REW (Room EQ Wizard) is an inexpensive way to do the test. Just download the REW software (it’s free) into your computer, connect the amplifier speaker output to the computer sound card "LINE IN" via a voltage divider, connect the "LINE OUT" to the amplifier input and you can start measure SPL level one channel at a time by using the software generated signal sweeps from 5Hz to 20kHz.

https://www.roomeqwizard.com/
https://www.epanorama.net/circuits/speaker_to_line.html

Yes, the correct way to do it would be with a signal generator and an oscilloscope. I believe this would be an unusual failure mode for an amplifier but hey, it is a strange world out there.  I see no reason why imhififan's method would not work.