Best used DAC under $750.

Without a doubt the giant killer DAC for even less than what you stated is your price range is the ifi Micro DAC2. I cannot believe what I am hearing with this! Buy it and save the extra money for a good cable. I cannot believe others out there charge what they charge for their DACs when this one kicks some serious rear....
Good luck!

I cannot believe what I am hearing with this!

Just curious what you compared the iFi to

Mytek Liberty and the built in DAC in my Marantz UD7007.  I find the ifi to be as good as the Mytek for a 1/3 of the price...

A used Musical Paradise mp-d2 mk1 just sold on USAM for $500.
A mk1 or mk2 would offer a lot of flexibility to swap tubes and even caps.
Audiocircle's disc less forum has a lot of great experimental info on this DAC.

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. 

Actually that is $699. Sorry for the typo. 

Topping D50. Measures better than just about anything else. If you want transparency, look no further.

Denafrips Ares II

A used Benchmark DAC1 HDR would fit in the ~$600 budget and has XLR out. I've had mine for about 10 years, use it as a DAC and preamp and have no complaints.

Three Things to look for:
1. Not over 5 years old
2. A Ladder DAC
3. Tubes
4. Volume control 

Smart to go budget now as these are improving quite quickly.

Keep us posted please. Hurray for #4.

How about a new R2R DAC for under $700.00? DENAFRIPS ARES is an incredible bargain. IMHO

I posted here for DACs I wouldn't even know about. You guys have given me several ideas to investigate. thanks

IF needing USB you will want to use a USB/SPDIF converter with either the aforementioned PS Audio DL III or my suggestion, the Bryston BDA-1.  The going used rate for the PS Audio is $300-350 or for the Bryston BDA-1 is $600-800. I have owned both, the Bryston is somewhat better and has far more inputs. Both have XLR and RCA outputs.  

Schiit Bifrost 2 is very good for the money.  have had mine for a month now and like it very much.

I'll be listing my Eastern Electric MiniMax DAC Supreme shortly for $595.00
I believe that it retailed for $1300. new. It has both tube and SS output.
I don't know much about the others that have been suggested here although I can safely say that the EE blew away the DAC in my Sony SCD-C555ES CD player. It has XLR out as well. Check it out.

Used: Schiit Gungnir Multibit Analog 2

New: Schiit Bifrost 2 (as recommended above).

If I was in the market, I’d get the Bifrost 2 and be done with it.

I have been happily using Bryston BDA-1 since 8 years. Currently there are atleat three offers on A’gone which can fit your budget ( under $700) comfortably.
Bryston gears are very reliable and long lasting. I would recommend to consider it.

If you are happy with redbook only,  then do your best to find a Wadia 15  It uses the Burr Brown PCM63 R2R architecture which beats, to my ears, the delta sigma DACs, unless you are prepared to pay huge money.

This on redbook is a more musical and rewarding experience than my Bryston BDA-1 playing hi-res.  The Wadia is built like a tank and can be found for about $700 to $1100

A friend brought his Theta over to compare. He went very quiet when he heard how much better the Wadia is.

Hello,

I’m about to upgrade and I have a Chord 2Cute which a great DAC. I will let it go for $600.00. You can send me a message or email me at tbrathw8@gmail.com

Jolida, now Black Ice,  FX tube dac for $550.  Awesome sound but no remote volume or input.    Search for the Arizona audio club shootout.

I second the Schiit Gungnir Multibit.  If you want MQA, find a used Mytek Liberty.

NAD M51

Oppo Sonica DAC, if you can find one.  No question in my mind as long as you are OK without MQA... which you should be.

The Jolida Black Ice FX Tube DAC DSD with WIFI is $800. The version without WIFI is no longer in production.

thanks for the offer on the chord. I am totally unfamiliar with them. I am leaning toward a liberty. thanks for all the suggestions. I have found out when you have questions ask them here and you'll get answers