i am a very happy owner of the denon d7000.
i recently aquired some shure se420 iems (returned, yuk!)

my question is regarding impedence/sensitivity specs and "hiss".

denon d7000 imp= 25ohms, sensitivity= 108db
shure se420 imp= 22ohms, sensitivity= 109db
very similar specs (as i see it) yet the shure's hissed like a pissed off snake, while the denons, whisper quiet.

i wish to buy some other iem's , and want to know how to avoid the hissing scenario?
cheers for any advice offered.

Well, as far as sensitivity ratings go, that's the output in terms of SPL at 1mW. I'd imagine it's not linear, though (nature abhors an integer), so while they have similar ratings, it probably takes less voltage to move the tiny little drivers in an IEM versus the full size cans. I am not a scientist, though, so that's just my (possibly flawed) logic.

I can tell you that that's fairly common when using IEM's (I really hate that term) with full-size headphone amps. You can either use a lower power amp, or just add some resistance to the line from the amp. Etymotic sells a cable that adds 75Ω impedance to the line. I'm sure there's a cheaper, smarter way to do it.

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the 420's hissed out of my sony a818, as well as my desktop amp(though not as bad).
i would not normally use an iem from my amp, it was purely to see how they faired.i already had other sq issues with them from my portable, best described as "lifeless".
a mate is going to lend me his klipsch s4's. if they prove better i will then buy a set for myself.
failing that will try UEtf10, although they are expensive.