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

Does that rule out all tube pre's?  System:

Hales T8

Ayre v-5xe and k-5xemp

MBL corona player

Not crazy about the idea of upgrading to the new Ayre Pre (K-X5 twenty)

russellrcncom Does that rule out all tube pre's?

Absolutely not!
My BAT is quiet as a tomb. I'd also look at an ARC Ref 5SE or, if you can find one in your range, a Ref 6.
If you buy used, you can get an Ayre KX-R (non twenty).
I just got one and even though it is only a few hours running, I can say it is unbelievably quiet.
For tubes, an Atma-Sphere MP-1 or 3 used should come in around $8K.
In fact, I will probably get an MP-1, myself.
My Ayre k1xe is quiet.....even with the volume up on phono, ear to the speaker.....nuttin.
Spread Spectrum Technologies (SST) Ambrosia.... Find any review,  I've sat in front of this and compared with many.... Around $3000 used around $10,000 new or $7500 new without phono.  Excellent phono by the way,  Excellent headphone amp,  balanced and single ended, tone controls, excellent remote and quiet as a tomb. 
My Supratek Cortese is silent.
What’s the efficiency/sensitivity of the speakers?

What is wrong with the Ayre KX 5 twenty?

See my system pics for the 6SN7 tube preamp that I build. Under $8K, brand new all point-to-point wired, with top parts quality.  I will offer a trail period also.  Dead quiet!!

Happy Listening. 
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.