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

How about an Audible Illusions Modulus 3? I want a remote, but maybe I could live without it...

You might be able to snag a Klyne at your price point. Excellent full function preamp.

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.

Thanks everyone. You've given me a lot of options to work with. I'll circle back once I make my decision and purchase something.

Daniel825, I think you are referring to the Parasound P3, not the P5. The P5 only just came out less than a month ago. It sounds like your experience was some time ago. The P3 was discontinued.