Personally, I wouldn't worry about double amping nor the cables. If your cables do pick up enough EMI for you to hear it, well, you'll hear it. It's a hum or a hiss. If you want to check, crank up the volume to max with no music on; if there's more hiss or hum with the extra cables than without them, then that's what you're hearing. (Don't forget to turn the volume back down afterwards!)
Amps are so good these days that one more or less in the chain won't make a noticeable difference unless it's defective, malfunctioning, or being pushed into clipping. And those kinds of problems are usually pretty obvious if they occur: hiss, hum, crackle, no sound at all, sound like it's coming through a heavy metal guitarist's amp. Or if something goes wrong exactly the right way, you might, say, lose bass. But I don't think hard-to-discern problems are all that common!
So unless something is actually and fairly obviously wrong, the chain from your (digital) source right up to the point the cable reaches your 'phones or your speakers is very robust; the signal will come through as good as undistorted, unless you intentionally apply distortion to it (e.g. with an equalizer or surround sound processor). The most likely potential problem is at that interface, e.g. if your amp can't handle the impendance of your 'phones or speakers. As @arnyk pointed out, this might actually happen with some Denon A/V receivers and some 'phones; if this is what's happening in your case it would explain the difference you're hearing. (It's also possible that it's just autosuggestion, of course; we humans are extremely good at hearing what we expect or want to hear.)
FWIW, I learned most of what I think I understand about this stuff from this thread
plus some related reading I did inspired by it. It was also an extremely entertaining read.