Output test on amp

No.

The problem with multimeters is they tend to be limited to a few hundred hertz.

Your easiest/cheapest solution is to use a cell phone with a calibrated mic:

https://amzn.to/2V3ahik

and appropriate software.  I use Audio Tools on android.

To see if a problem is an amp or a speaker, swap speakers.

Of course, you can also get really fancy and go with Room EQ Wizard or OmniMic.

All power amps (and preamps) have linear (flat) response out to their upper and lower limits, where they roll-off in the ultra-sonic and sub-sonic regions. So no need to worry! 

Many amps actually go right down to DC. The upper limit is peculiar to that am and it's design but I no of no amp that does not pass by 20 kHz gracefully. What is it exactly that you are worried about and why?

I would get a digital test album like this one  https://www.hdtracks.com/#/album/5ddb5ef3848da9f501f6db95
Then you will need a broad bandwidth multimeter. This will check everything but your turntable. For that get the Hi Fi News Test Record. 

I have a 30 year old ARC CL 60 that appears to be a few decibels down in the higher frequencies in the right channel.  But it’s subtle; so I’m not 100% sure. 
I have eliminated all of he other variables including,  it not limited to, cables, tubes, flip flopping speakers - and tubes and sources. So I figured I do an empirical test before sending it in. 

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.