I think this will lead to a very subjective debate on the matter. Personally, from what I heard through a pair of Ety 4Ps, I have to rate the hiss 3 to maybe 3.5. Do you really have to get a mp3 CD player right now? In a similar position, I'd wait for a while until either Panasonic or another big name releases their version of a mp3 CD player or until the SlimX hiss is addressed via a firmware update. You might also want to wait for Sony to release a newer player with LCD remote.
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 for details.
see my review of the slimx hiss revisited...
it seems to come and go with the media you are playing...
I don't think it's the player, but the cds or recording media.
I don't work for iriver. I just think this hiss thing is unjustified!
Sometimes it's there, sometimes it is not. You be the judge!
Again, I am ordering a TAH amp and perhaps my review will change, but through grado sr60 headphones, the hiss seems to come and go.
hope this helps,