I own the Original Master and use it in my office rig as follows:
FLAC>iRiver H140>Lavry DA10>OM>W5000
I have also tried the Headfive.
I believe those are the two great bargain in the <$500 market at the moment.
- The Original Master has a very neutral sound signature: it actually maintains the signature of your source. The bass is very strong, but also very tight and very much in control (I think due to its generous separate power supply); it never struggle for power: no distortion at all, even in fast and complex passages; sound is surprisingly crisp and detailed, no grain at all.
I believe it has been voiced for the HD650 and it pairs particoularly well with it, but I use it with good results also with the W5000 (low impedance).
As I said, of all the amps below $500 that I have tried, none gave me the wish to upgrade (I am not necessarily saying it is the best <500, but no other is noticeably superior).
I would definetely recommend it if you like the sound of your source. On the flip side, do not expect an exceptional build quality, but for the price can't have everything.
- The Headfive impose its own sound signature to the source: It has nice round, silky sound which remind me a bit the lushness of the classic tube amps. It would be the best choice if you don't really like the sound of your source, although I must say that with the Lavry as a source I prefer the detail, crispness and tight bass of the OM.
As usual, if you can give them a try before buying, but you certainly are on a good track
Hope this helps.
Vic
PS: The US importer of the OM is:
http://www.aaa-audio.com/index-library.htm