I have the Monolith usb dongle dac that is part of this thread’s title. It’s been awhile since I’ve done anything with it but I think it is designed to have similar specs to the DF red. I have vague memories of it driving the 6XX decently. I’ll see if I can find it and report back on that…
I got myself a BTR5 from Fiio … you can easily pair your laptop with it via BT, and it’s amazing when you’re on a go with your phone… plus: I can even pair it with my living room LG TV and sounds amazing!
I love it and recommend to anyone 
how did you feel about the monolith… seeing as it has more features except for MQA and costs less
It’s ok. I’m trying to remember why I bought it. I think it went on sale for something like $60 on a Black Friday sale and I ordered on a whim after 2 fingers of Maker’s Mark. It’s certainly better than a phone’s or PC’s headphone output. I just checked again, it has plenty of power to drive the 6XX. It did a serviceable job driving a 600 ohm DT880 to decent, thought not necessarily fun, levels. There’s nothing really wrong with it at the price point, I just had better gear already and it didn’t surpass what I already had.