Which used preamp for Halo A21, under $1,000?

Agree with the Muse Model 3.... excellent, single ended, balanced and remote.... Look for the 3 Signature, an upgraded version, should still be had for $800 to $1000

If you used it in Balanced mode, the Audio Research LS 9 can be had for $1000 to $1200
Coda Window's Continuum 3 or 4 might be found at that price.
Adcom GFP-750 can be had at that price
Maybe a Classe 5 or 6 (not DR, Not 50 or 60) DR no remote...50 or 60, most likely more money...
These are all fairly neutral, all are balanced and single ended, all have remote and all will be worth what you pay for it in the next few years.

The CLX is a Coda CL with upgraded parts. The CL is a slight improvement over the Windows Continuum 4. I own a CL, it is the highest value pre that I have sat in front of. This is a bold Statement, but I would have no problem putting the CLX in the league of a Pass X10. Very high performance for the $$$. It really depends on your taste, but certainly worth a listen, but....How do you get a CLX for $1000?

I am in St. Louis, I've had several established audiophiles sit in front of my system and have heard comment after comment about the transparency of my system... Cables made a difference, but can't comment on the reviewers experience. You would certainly be welcome to my home for an evening of listening.

Louie,  take a look at this thread that I did just recently,  its not very long to read through,  it may help a bit. 
Tim

https://forum.audiogon.com/discussions/should-i-switch-preamps