Well, JH13Pro, ACS T1, CT6, and the Earsonics EM3Pro - the Jerry Harvey are in some ways the least sensitive. When music is playing, I doubt you'd be bothered by it. I am because my ear is very sensitive. The iPod hisses less, but the S:Flo T50 isn't bad; it is probably on par with a Cowon D2 if not a bit more. iPods (current ones) tend to hiss the least of any players, so take the fact that the S:Flo hisses more as heftily as you would a pillow - it is simply par for the course.
The warbling in the background is steady unlike the Rocoo, so it becomes the background, but who knows.
The JH16Pro is probably as good a choice as the JH13Pro is for a lot of applications and just won't hiss much.
So far, I really like it, but am not ready to place an crowns on its head or anywhere above its shoulders.
I also do not hear any actual hiss like you get from some cheap portable amps or some players. The only artifacts I hear in the background is the slight warble you described but no hiss at all using my Hippo VB's which have pretty extended treble.
Teclast has said the warble is due to a voltage step up when the screen is on but I think it happens even when the screen is off.
So I would say it is a slight issue with the warble but from my tests it actually is not constant during the music and there is no actual hissing from the HO.
The issue of warble does not happen with the LO. But even with the warble at the beginning and end of some tracks I still prefer the HO to the LO as the LO is to loud and to cold to pair with my XM5 amp or to plug in directly with my IEM's.