I've had my set of 650's for a mere two days, and I'm also using the Audio-gd Fun as my DAC/AMP. I've got an April 2010 revision A version of the Fun, and previously listened via Alessandro MS-1's, and Denon D2000's. I have always enjoyed the MS-1's and use them almost daily for podcast (voice) listening, for music they are nice as well, but lack bass. The Denon's are fun headphone, but are extremely bright, sibilant and in some cases just downright painful to listen to (for me at least). The Bass however is awesome. I've got all 3 DAC's for the Fun, but the WM8741 is the only one I'll use going forward, the need to solder it in doesn't facilitate going back to the others, but it's also the best of the 3 to my ear.
The 650's were simple to switch to, just releasing the gain button on the Fun allows for plenty of volume at the 9 o'clock volume setting. Some older more dynamic recordings need a bit more volume. Out of the box the new 650's were a bit muffled, now I've got maybe 10 hours on them, and they sound fantastic. Enough bass, nothing like the Denon's, but enough to satisfy, and it's a well defined bass. The upper frequencies are very present, I did some A/B with the Denon's today, and they certainly are not as bright or sibilant, but the treble is well defined, I wouldn't call it rolled off at all. These phones are a pleasure, comfortable, with an extremely smooth sound. For me the Fun and HD 650 are an excellent pairing. I haven't tried the Moon HDAM yet, but eventually I'll find time to do that. For now the WM8741 with Earth is just excellent. I've got some time off from work next week, and will get in some listening sessions. I'm looking forward to it.
The only downside to all this is my USB input of the Fun no longer works, windows cannot detect the USB device. It worked find up until Tuesday of this week. I put in an order for the upgraded USB input with Mr. Kingwa before it stopped working. I'm not sure if it was static discharge, or even a cable getting pulled, but it's won't work. It's put a real crimp in my listening, as I usually use computer to USB as my source, for now I'm playing my audio cd's in my old CD player. I got that player in 1992, and it's on it's last legs, it has difficulty tracking and skips from time to time.
RonO