Hi, I have 3 components that I use with my onboard sound (ASUS P5Q PRO):

Behringer MS16 - pretty awesome studio speakers at an affordable price.
Sennheiser HD500 - had these awesome headphones since 2003, but they're starting to break apart, literally.
SteelSeries Flux - bought these headphones as a replacement for the above.

What I'm experiencing is a bit weird:

• Motherboard -> Behringer MS16 = Sound OK
• Motherboard -> Behringer MS16 -> Sennheiser HD500 = Sound OK
• Motherboard -> Behringer MS16 -> SteelSeries Flux = A lot of noise, sound not OK
• Motherboard -> SteelSeries Flux = Sound OK

How come Flux input directly into motherboard is ok, but put into the Behringer MS16 it fucks up? And Sennheiser HD500 has no problems with being connected through Behringer MS16.
I can't figure out why this happens?

Probably can't drive the low impedance of the Steelseries headphone.

Sennheiser input impedance = 150 Ohm
Steelseries = 29 Ohm

My understanding is that impedance problems usually lead to loss of bass, not increased noise. My guess would be a ground loop. Try plugging the headphones into the speaker through an extension cable. If either the HD500s turn dirty or the Fluxes turn clean, then that's almost certainly your problem.

If the headphone amp inside the studio speaker isn't designed to drive a low impedance load like those Steelseries, the net result could be what the OP heard.

Ah. I was assuming that it was just hiss/crackle/hum the OP was hearing, which he did not explicitly say. I'm also more knowledgeable about high-end tube/discrete headphone amps, which act a bit differently from opamps when pushed beyond their limit.

A lot of what you read on the web about headphone impedance is misinformation, sometimes straight from swindlers who sell overpriced products.

What is likely happening here is that the *sensitivity* (not impedance) of the headphones is different.
The Behringers likely have low-level analog noise on their headphone out which you may not hear on insensitive headphones (the noise is still there, just at a lower level). I've used insensitive headphones to attenuate analog noise many times.
Insensitive headphones are however a poor match for crappy power-starved devices or simply deliberately crippled ones (so that people won't blow their ears out by plugging in sensitive headphones and dialing the volume to the max).

A more precise description than "sound not OK" might of course invalidate my theory.