Do High Impedance Phones hiss when used with an Amp?

Try different outlets or a cheater plug (plug without a ground), it might be a ground loop.  wait, nvm... you're not getting it from the K240? hhmmm maybe not a ground loop then. 

Are your RCA cables directional? If so, check to see that you have them in the right direction, and then correct them if not.

The output of the amp is at high impedance if it is hissing.  Typically headphones around less than 32 ohms that have high sensistivity, will cause hissing because of the impdedance imbalance.  Is there impdedance setting on the amp?  If so, try lowering it low as possible.  Or maybe get an impdedance adaptor to increase the impedance of the highly sensitive headphones.

Id bet those are not directional.  You actually have a good enough entry level system and resolving enough headphones that a good set of RCA cables should make a noticeable improvement.  Your ears might just be sensitive like mine are.  I recently had to sell an amp because I could hear the background hiss.  Drove me nuts. 

Did you get the x-head new or used?  The first version of the x-head was supposed to have a shielded transformer but got built with an unshielded transformer and that caused some hiss.  The second version of the x-head got a redesign and a relayout of the circuit board to fix the hiss.  The second version should sound clean and free of hiss.

The second version has a sticker on the bottom that will say "Rev B".

If you use the x-head with no source connected do you still hear any hiss if you turn it up?

I have an x-head (rev b) and it is quiet, no hiss with my headphones.

I can't put up with even the slightest hint of hiss or noise coming from an amp.  If I'm gonna spend decent money on a headphone system I want a clean signal with a black silent background.  

In my experience tho it is low impedance headphones which are usually the culprits, not the high impedance phones.

Couldn't agree with you more.  Its why I just sold my last amp and am going to be buying a RSA XP-7, which is said to have a black background. 

I think we need to differentiate "hiss", from "hum".  If we are talking about "hiss", then this is not a grounding issue.  Let's assume that is the case.

Some amplifiers have enough intrinsic noise that if you have a very high-sensitivity headphone, you might be able to notice it with no music playing.  With almost any amp, if you crank the volume all the way up to the top, you will hear some hiss through almost any headphone (again with no music playing).

Further, some SOURCES will have enough noise that this will be audible though the amp - and this is not the amp's fault!  Also, some amps will hiss when there is no source connected, but won't when the source is connected but turned off (this is an amp topology issue).

Finally, a very small amount of hiss may actually be a good thing, in that the way this is supressed sometimes is by using global negative feedback in the amplifier circuit, which many people (me included) believe had have a negative impact on the sound.

Bottom line - it's pretty common to have some level of hiss.

I guess I mean audible noise.  With my Burson amp there is no noise or hiss (audible to me) even at the highest volume level with no music playing (with any of my headphones)