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

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.

So as I continue to hunt for the pres recommended in this thread, I've been thinking (always dangerous) about going with a tubed unit. Any thoughts on the Parasound Halo A21 with either an Audible Illusions Modulus 3A (used) or a PrimaLuna Prologue Three (new)? In both cases, I'd be sacrificing a remote, but I might be willing to do it if the sonic rewards are there.

your amp will do best with balanced preamplification.