PREAMPLIFIER UPGRADE QUESTION

My choice would be an Aesthetix, Calypso. Newest version at $6K is super quiet, fully balanced and with just (4) small signal tubes, a rollers delight. We all recommend what we use or have experience with and the Calypso, for the money, was a clear winner for me. 

Great selections but I'm biased to the Balanced Audio Technology sound. Have a look at BAT VK-80 over at music direct. It hits the budget you gave

Buy the Zesto.... you can thank me later !

I bought a used Ayre Acoustics KX-5 Twenty preamp for just over $5K.  Fully balanced and dead quiet, it is very dynamic at lower volume settings.  I can't believe what I was missing before.  I've got it paired up with a Parasound JC-5 amplifier pushing a pair of Infinity Kappa 9's.  There's a lot out there to choose from because many people are dumping expensive preamps in favor of bypassing and using a DAC with a volume control.  I'm not in that camp, get a good preamp like the KX-5 Twenty you won't regret it. 

If you love your Martin Logans look for something that plays well with your electrostatic speakers.  I had McIntosh and it did not work out very well sent direction I had to go in a differ 

Good Luck and let us know how you make out!