Best used DAC under $750.

A good, used Ayre Codex might be your answer. It is near that price range used, or slightly above. Also has XLR outputs.

If that was my budget, I would own a Mytek Liberty.
fully featured.
MQA support.
volume control.
headphone amp,
good US company.
very well reviewed.
good form factor.

have seen them used within your budget.

I purchased a used PS Audio DLIII Digital Link from The Music Store this past year and have been very pleased with it. I believe right around $360 shipped (which is pretty typical). It also has XLR outs.
I run both my CD’s and all streaming through it, and the improvement over any internal DAC’s I have is ‘night and day’.

I wasn’t sure I needed a separate, but the DLIII convinced me I do. When I first started shopping for a used DAC, my impulse was to find one with ‘newest shiniest most popular’ chip. Then I listened to Paul from PS Audio explain it wasn’t necessarily the chip that will create the best sound, but instead, the engineering of the output stage. The DLIII has a Burr-Brown chip, but I was impressed by Paul’s insistence that a DAC with the newest chip but a poorly designed output stage is not the way to go, so gave this old DAC a try.

Sometimes you can find these with the Cullen mods, but those typically run between $700-1,000, if you can find one.

Within your budget
https://wyred4sound.com/products/digital-converters/clearance/dac-1v2-trade

You can get a new redesigned Schiit Bifrost for $669 plus shipping. To me that would be a real contender.