What does clipping sound like?

Ok, you didn't provide much information such as how old your components are, how many hours on the tubes, how old is the cartridge, was it set up properly, etc. If this is a new and unchanging occurrence, I agree that mis-tracking is the most probable cause and a great place to start looking. The easiest way is to simply try another source and see if you still get the noise. Once you rule that out, consider tube replacement. Good luck!

From what you describe, it is NOT clipping.

IF it was clipping, it would occur no matter what source, and only at high volumes.

60 wpc/91db/6 0hm is a fairly easy combo unless going for a lot of volume, likely unpleasantly loud before clipping.  

IF only TT, there you go, it's NOT the amp, NOT too little power.

You may want more power, have long thought more power would be nice/better, but don't let this problem go unsolved before moving up in power.

1. prior to alignment issues, check if you are sending too weak or too strong a signal to your preamp's input. what cartridge? what preamp?

2. TT arm/cartridge balance/alignment: how are your skills?

91 dB 6 ohm nominal load speaker, generally speaking, should not be that hard to drive unless the impedance has really low dips.  Again generally speaking, tube amps tend to be able to provide more current than similarly priced SS amps.  So, depending upon what tube amp you are using for these speakers, it doesn't sound, to me, like this is an amp problem, per se.  However, as one commenter suggested, it might be a bad tube.  Maybe it's time to roll out those tubes?  If that doesn't do it, I would check and re-check all the basic stuff (e.g. connections).  Maybe this is a speaker problem.  If not, my next attack would be a good power line conditioner.  Good luck!

I don’t have this distortion on Netflix, etc.

So its not the amps, nor is it the line stage of the preamp. Its something to do with the phono.

All TT settings were done with the guidance of Tri. Believe them all to be correct. Think it must be a combination of mistracking and bad, old records.