Used Pre under $8K with lowest nosie floor(?)

ozzy3:  the ayre combo is a little lean in my opinion and I'm not sure that the KX-5 would be any different.   

I have recently acquired Atma Sphere MP-1 MK 3.1 which I'm using with Atma Sphere M-60 monoblocs and Quad ESL 2912 speakers. MP-1 is absolutely fantastic machine (as well as M-60). If you are looking for new preamp, you mush listen this preamplifier. It is expensive but this is one of the finest preamplifers ever made and you'll have a lot of troubles to find better at any budget.

What do you define as low?  

If you are using single ended connections (RCA) the best SN ratio you can achieve with a standard 2v output is 100db.  True (4v) brings that to 125db.  20v professional balanced can push it to 135db.  

Your SN ratio is only as good as the SN of your weakest component.  Ayre doesn’t list SN ratios.   

Schiit Freya+ measures 104db sn balanced and uses a tube.  
If you want bragging rights, the Benchmark HPA has a SNR of 135db balanced.  

Both of the above likely well exceed the SNR of your amp.  I would say the more important question is what you don’t like about your current preamp and what you hope to get from a different one.