With the slimX, I find the hiss is not volume dependent. With my HD590s, and a home built cmoy amp w/ linkwitz crossfeed, I can totally eliminate the hiss (bring it down beneath the noisefloor of the room) by turning the slimX up, and turning the amp down a few notches. Totally fine. If i had complete isolation, i'd still get a bit of hiss, i can imagine, but with my 590s, it isn't a problem at all.
If you want better quality audio, you should go with a regular CD player anyway, mp3s aren't for you. But I think the slimX has quite good quality for what it is, and is feature rich enough to be well worth what I paid for it.
BTW, they just lowered their prices by 20$, and are even sending 20$ off coupons (for any of the accessories, extra batteries, cases, etc, in their store) to people who registered before april 15th. If you bought before april 15th, but haven't registered, they are taking late registrations with dated receipts, see www.iriveramerica.com