Clipping

Can’t find Fritz Carbon 8 MK II online. Need to know the impedance as well as the 84dB spec.
+1 roberjerman.

Also, have you tried different a different load setting on phonostage?

phil, turn the volume down. If you still hear the distortion it is most likely your cartridge miss-tracking in which case you have to go over your tone arm set up. Koetsu's are not the best tracking cartridges. It could also be the cartridge overloading the phono amp which I doubt unless there is perhaps a bad tube or the phone amp overloading the preamp. If the distortion goes away with lowering the volume then you are most likely clipping your amp. But you said the volume was not that high so I have a hard time with that theory. 

@lowrider57  did not reread my email. Speakers are Fritz Carbon 7 MK II speakers..88. Db

OK, so they're 8 ohm nominal.
I don't see an impedance curve, but Fritz states that they are easy to drive with low powered amps. Very little chance of clipping.

I'll agree with others that it could be cartridge or phonostage related. I've played lots of Zeppelin over the years at high volume and never experienced clipping.

" This particular record is famously distorted and is intended to sound this way"

+1.  It is pretty rough sounding, in a good way, regardless of pressing.  Lots of overdriven, distorted Bonham for example.  When he comes back in hard on Whole Lotta love it is a lovely mess!  I'm inclined to think you are perceiving the album's distortion as clipping.