Upgrade from Adcom GFP-750?


Having recently added a Bryston 9BSST, I'm thinking about upgrading from my Stan Warren modified Adcom GFP-750. I've been pleased with my Adcom's sound, flexibility and reliability, and I think I'll be even more pleased when the modified unit arrives. But I'm wondering if and how much better I could do within my budget (~$1500, used). I'd prefer to stay with SS. The main contenders are the BAT VK-20, Bryston BP25, and Conrad-Johnson PFR. Would any of these, or others not on my short list, represent a significant upgrade in sound quality? I appreciate your comments and suggestions.

System:
Adcom GFP-750
Bryston 9BSST
Linn Ikemi
Thiel CS2.3
Kimber KCAG ics
Acoustic Zen Satori cables
lmack
I'd hold off until I let the Stan Warren mod burn in for a few weeks. The Black Gates he uses have a long "gestation period." Other writers have suggested good options that will always be there.
You're right, Ivanj, and that's what I'll be doing. But I still like to explore the possibilities, in case something very attractive comes up used. So far, I like the Pass and Sim possibilities, even if they would mean stretching my budget a bit. Any more suggestions would be most welcome, and thanks all for the suggestions so far.
Honestly Lemack, For the price you mention,I cant think
of any preamp that can beat this.When I replace my silver
cable to cardas golden cross balance, every breath of this
unit came out, no wander this unit is a Stereophile Class
A unit.Bryston wont go near this adcom. Try to use the
balance and try it will full sounding cable like cardas,
audioquest, because this preamp if you put silver it will
give you leaner and slight brightness.You dont even need
to spend $1500 if you hit jackpot on the right ic,just
be patient.