Just got one of these today and I've pretty much spent the day spinning SQ discs! The front-to-back separation is unbelievable...the front-center vocals are almost fully suppressed in the rears.
I've particularly enjoyed revisiting those few Columbia SQs that haven't had a discrete release- Turnstiles, Native Sons, Energy To Burn, etc. It really brings out every last bit of separation in the B.T. Express album- that may become a demo quad disc for me.
However, one weird thing I'm noticing is that the unit seems to be better at cancelling any element isolated in the right rear position in the fronts compared to left rear. I first noticed it on Jeff Beck's "Ice Cream Cakes"- listening to the fronts isolated, the lead guitar in right rear is almost entirely erased while the piano/keyboard in left rear is suppressed, but still audible. I then noticed it again in Loggins & Messina's "Angry Eyes", with the sax solo in rear rear wiped from the fronts while the bass in left rear is still somewhat audible. Then again in Dave Mason's "Baby...Please", where David Crosby's right rear harmony is suppressed while Graham Nash's left rear harmony is still somewhat audible. You probably get the point by now...
It's really not a big deal- it's not noticeable at all when listening to the unit in the sweet spot without isolating channels. But it seems strange to me that the front-to-back crosstalk is greater on one side vs the other...I hope it's not a hardware-related issue.