My main gear currently are an Ifi Zen DAC and some nice Hifiman 4xx plugged into them.
No matter what, I keep coming back to these two. I have other gear, not this is the one that pleases me the most. It is not my endgoal, but it’s superb to my ears.
I got the MassDrop 6XX for Christmas a couple of years ago and heard they were basically a 650. When a great used deal came up on the real thing, I grabbed it to do some A/B comparisons. Yep. LOL. Pretty much the same.