As long as you are not looking to fix any problems with the upgrade (a good thing in my opinion), then here's a few recommendations.
Solid State: Ayre K5, an ML to match your amp, and maybe a Rowland or McIntosh SS, but with either of those you'll really need to try them in your system first. The Ayre would be my first pick.
Tube: Aesthetix Caplyso, Whatever BAT is in your price range, same thing with ARC but its a must audition in your system. The Aesthetic would be my first pick.
Due to the equipment you list, I left out anything single ended. Also, have you ever tried connecting your Esoteric directly to your amp? I know that you may not want to go without a preamp, but depending on how it sounds direct, may help you select the right preamp.
