Shrill high frequencies

At your suggestion I changed the power cord from that provided to Shunyata Delta. No joy. Maybe break-in time will help. 

Maybe the Isotek needs more break-in time. 

BTW - I use a MD 108T + KEF LS50 and those 2 are definitely not bright. My Accuphase T-101 tuner is much brighter than the MD 108T

I have the Isotek sigmas unit.  I did a power cable shoot out with my amp, and found that Shunyata cables did not like being plugged into the Isotek.  Especially the NR versions.  Although I did not have issues with high frequencies, in my case it was a tonality issue.  I plugged the amp directly into the wall.  Even with other PCs, I recommend trying plugging the amp into the wall.  

Try MIT or Transparent cables and IC's instead of the Cardas/Nordost Frey. That should do it.

Matt

Did you try the Isotek PC that came with the unit?  I too have a Sigmas and have noticed no brightness at all.  I use a Synchro with it, but a regular PC would be fine.