Yes, I used a Dangerous Source for many months.  Beats the Benchmark Dac 1 easily to my ears.  I like it better than lots of under $750 or so dacs and prefer it to the Eastern Electric Minimax, Zdac, Blue Circle and others.  Moving on up to something over $1k though and it gets outclassed if you select the right dac. 

Headphone section is just okay and a bit better than the dac 1.  Not quite up to the standards of most good solid separate HP amps, though.

Thanks very much. Any thoughts on what would be a good upgrade DAC and amp for the UERM specifically (no need for it to be portable)? A fairly compact unit would be good, as would pre-outs on the DAC or amp so I can use speakers too. Similar to the Source's features, I suppose, although I don't mind separates. Not looking to spend too much at this stage.

... yes

on the topic of audiophilia nervosa:

If you have ample time(maybe during drive to work, jogging etc), do listen to these presentation. Really practical and potentially money saving(audiophile wise) advice/tips from him.

e.g. at 31m:32sec of the RMAF13 video, he recommends you to set aside time to listen live music, so that you can judge your home audio equipment's sound quality better.

Well designed R2r dac does less evil than well designed sigma delta dac.

How many of you are unable/unwilling to use delta-sigma DACs once you own an R2R DAC?

I'll be getting a GMB, and I'm wondering how I'll find using the M-DAC + Valhalla 2 + HD650 combo as my secondary rig.

I have a gumby en route, so I can't comment directly until next week, but nearly everyone here I can think of that has (or has experienced) a good R/2R dac (Yggy, gumby, theta data, transdac, etc), also has (or is soon to have, or has listened to and commented favorably on) a Geek Out V2 (whatever iteration). Since the GOV2 uses d/s sabre chip, I can only guess the answer is "no one". Like balanced / SE amps, its mostly about implementation than dac type. I am not suggesting specific performance comparison; I just highly doubt that listening to good/great R/2R will make all d/s unlistenable.