Even if my HD650s aren't burned in yet, I simply couldn't resist and just gave it a try. I attached it to the line-out of my Iriver SlimX 550 and tested it with one of my most demanding cd's in terms of dynamic range...a Chandos disc containing Stokowsky's arrangements of some Mussorgsky's pieces played by the BBC Philharmonics...
Well, even if the SlimX certainly hasn't an audiophile sound quality (but it still sounds pretty good...so good that Headroom recommends it as portable cd player), the XP7 presents himself with absolute silence... no hiss or other artifacts at all! The sound is cristalline and (of course) much, much fuller and better placed on the soundstage than it was straight from SlimX's headphone out. There is no hint the amp is approaching its limits even in most demanding climaxes (I think anyone who knows this disc can confirm that some towering crescendi and climaxes with full organ pedals, grancassa and tam tam could give some problems to some cheap amps).
I think no one should worry about the XP7 being capable of driving high-end Sennhesers from a portable device...my SlimX 550 was set on 28 (max. is 40), and it was loud enough (comparable to the volume level you would hear in a GOOD concert hall...that's what I want relating to volume, not more and not less) even without pushing XP7's to its limits! Shouldn't this be enough for some of you, you can simply set both SlimX and XP7 to max...still, perfect sound with no hints of artifacts or loss in detail! Simply awesome...
Will post more comments after the burn-in period ;-)