Looking for a preamp...............

Boy does your system bring back memories!!I had the same pre and amps bi-amping Paradigm Monitor 7's.It served me well for some time.I then went to a Rogue TempestII Magnum with Tyler Acoustics speakers and never looked back!!To answer your question on pres, I replaced my GTP-400 with an Adcom GFP-710 about a year before I went to tubes and it really improved the sound of my system.You can find these for around $200.00.I highly recommend this pre with the 535's.Good luck.

I also had the Adcom preamp/tuner some time ago. I went to a Quicksilver tubed linestage and to this day believe that was one of the biggest improvements I've heard in any of my systems.
At the time I had the Adcom pre, a NAD 214 amp, and Paradigm mini monitors

The already mentioned Quicksilver would be a very good match with your other system components.