Need help in trying to trace cause of distortion

I second starting with headphones for troubleshooting  That is my first goto.
If you have none I recommend 7506's
https://www.amazon.com/Sony-MDR7506-Professional-Diaphragm-Headphone/dp/B000AJIF4E/ref=sr_1_1?keywor...
Not great for extended listening (bright) but good analytical monitors.
However not recommended for dry environments.  They will static zap your ears. I switched to Focal Listen for recording after 25 years of anti-static spray in the winter.
A good enough complimentary amp is
https://www.amazon.com/Nobsound-NS-08E-Integrated-Amplifier-Headphone/dp/B014FASL1A/ref=sr_1_3?keywo...
It sounds better than my Topping TP21 and Schiit Magni 2.
Especially if you roll the tubes.
The whole sets you back $150.

Does this distortion come from one speaker or both? This would be a good starting point.

What makes this problem hard to pin down is that I’ve heard it while streaming a few times and stupidly don’t remember where.  I’m going to have to check through what I’ve streamed to find it. 

First thought...

-make certain that the distortion is actually coming from the speaker/speakers and not something vibrating in the room

Second thought - if it's coming from the speaker/speakers... 

-gently check the exposed screws on all the drivers for loose ones

Third thought...

-your preamp has a rapid/measured (Stereophile) increase in distortion in the 1000-2000Hz region that quickly spikes down afterwards.

The 3rd is highly unlikely, and I assume that the spike may be dependent on the tubes used when measuring the unit, but it's STILL there in the review.

I once has an intermittent "distortion" with Castle speakers, and it was emitting from the speakers.

Think I described it @ the time as a sound "accompanying" the music, but not a part of such.

The remedy ended up being the placement of small felt circles over the mounting screws of the woofer as they were dome headed screws in the sound path of the tweeter.

DeKay

Hello rvpiano. Let's start at the beginning. Be sure all the connections are tight and solid. Swap the speaker wires at the amp. The right channel is now connected to the left speaker and visa versa, right? Did the problem switch to the other speaker? If yes, it's NOT the speaker. If the problem is still in the speaker in which you first noticed it, it IS the speaker. If the problem moved, swap the wires from the preamp to the power amp. Did the problem move? If it moved, the power amp is OK. If not the power amp has the problem. You didn't say where the music is coming from. CD player? Phono cartridge? Where ever it's coming from, and the problem has moved every time you swapped wires, swap those wires that carry the signal to the preamp. Did the problem move? If it did, the problem is in the signal source. It could be as simple as the phono cartridge is not trackng the groove. Clean the record. Give it an extra quarter gram and see if that changes things. If it's a CD player, you know it's acting up. If the problem did not move, the bug is in the preamp. Happy testing.