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

The Quicksilver preamp is one of those products that works well in a variety of systems.

The ARC LS-3 might be too unforgiving.

You should take a look on Audiogon for an Audio Research LS-3. Years ago it had a class A rating by Stereophile. They sound good and can be had for $500.00. If you go up from there I would by a Sonic Frontiers SFL-2 or Line 2.

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

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

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.