Hifiman He6se $470
Hifiman refurbished with warranty (12 months or like them on Facebook to extend it to 18 months)
$509 and coupon code 39OFFRB $39 off refurbished for $470 with free shipping and they never charge me sales tax
https://store.hifiman.com/index.php/he6se-v2-refurbished.html
Hifiman Ananda Stealth $349 Hifiman refurbished with warranty
https://store.hifiman.com/index.php/ananda-stealth-refurbished.html
Shenzhenaudio Topping A50 III + D50 III + RCA Cable Combo $433
https://shenzhenaudio.com/products/topping-a50-iii-d50-iii-rca-cable-combo
eBay listings aren't mine to avoid a conflict of interest:
Hifiman HE-500 $429 a pretty rare bird to see available at all
https://www.ebay.com/itm/166666472015
SMSL D-6S balanced DAC $145 free shipping
https://www.ebay.com/itm/364804865304
Drop Hifiman HE5XX $105 free shipping. I use one for a daily driver, so comfortable with medium clamp.
https://www.ebay.com/itm/235487115606
Topping G5 $180 used, Hificollege is great about their warranty on used stuff.
https://www.amazon.com/KGUSS-Topping-Portable-Headphone-Amplifier/dp/B0B9RZ9STC/
Topping A90 Discrete Silver $400 used w their warranty also
https://www.amazon.com/KGUSS-Discrete-Headphone-Amplifier-Pre-Amplifier/dp/B0B4JRY19Z/
There's a black A90D from Aoshida-US for $419
I haven't tried their warranty yet, so don't know if you have to ship it to China. Hificollege is in the U.S. for repairs/exchanges.
https://www.amazon.com/A90-Discrete-Balanced-Headphone-Amplifier/dp/B0B4N7RBXX/
SMSL HO200 $199
https://www.amazon.com/HO200-Balanced-Headphone-Amplifier-Amplifiers/dp/B09D7T9KW8/
I've had these and really like them. I tried sub 12hz content on a DT990 600ohm and couldn't get it to go into protection mode, so I think the issue on ASR is probably only extreme cases (or maybe didn't have them loud enough).
S.M.S.L D400ES Audio DAC $439 used from SMSL Factory store so they warranty it. I have a D400 coming Wednesday, if it sounds as good as the DO300EX I have, but with 3 watts into 32ohm instead of 1, will keep it. I'm sure this is a good DAC though, the D400 on sale is probably the best value.
https://www.amazon.com/S-M-S-L-D400ES-ES9039MSPRO-Decoding-Bluetooth/dp/B0BLGWBPH3/
SMSL M300SE $103 used from SMSL Factory Store.
https://www.amazon.com/S-M-S-L-Upgraded-Headphone-Amplifier-Bluetooth/dp/B0C4DFNCSV/
It's nice, but if you can swing it, the DL200 is at $151 and I just got another one last week and keeping it for a daily driver. The 1.5watt into 32 ohm amplifier is really good, on the M300SE the 4.4mm isn't bad, but you'll get frustrated and add an amplifier to it eventually for headphones.
SMSL DL200 $151 from S.M.S.L. Factory Store via Amazon shipping
https://www.amazon.com/S-M-S-L-Headphone-Amplifier-ES9039Q2M-PCM768kHz/dp/B0CCY9MXMD/