I can't rule out the sound card, since it sounds bad via headphones, unless all headphones are faulty (quite unlikely). Besides, what can go wrong inside a headphones set ? They only have basically 2 electric devices, 2 coils. Passive components. They can either have a mechanical failure (coil rubbing the magnets or membrane broken) or a wire failure due to excessive heat melting the isolation or simply discontinuity. Not the case. Why do I say it sounds better via receiver and passive speakers ? Because somehow they tend to "enhance" the sound somehow, both receiver and active speakers have all sorts of filters, EQs and so on, a set of headphones doesn't have. Plus, headphones are connected in close vicinity of your ears, you spot faults easier.
I didn't test the receiver extensively at all sounds and levels and sources, because so far, I didn't have to. When I bought the card, I plugged it into computer, connected the headphones on front panel, it worked fine, then connected the sound card to receiver and I used headphones on receiver, it sounded even better, perfect (to my likening) I should say. Then I moved and unpacked the computer at first, without receiver, passive speakers (which are big and heavy) and other big devices and decided to use the front panel sound headphones plug on the sound card for a while. Result was bad, somehow, during my listening via receiver, sound card went bad (and I failed to notice). Or perhaps during the moving of computer. So, I stopped using receiver and passive speakers to avoid inflicting damage to it. I bought this set of active speakers and connected to the coaxial output.
I know the thread title is about "capping", but I am more concerned at the moment about the general sound I am getting via headphones, more than capping. I am sure the active speakers were faulty when I bought them, with this limitation of sound somewhere. If the capping is happening inside the active speakers, at least one problem is solved. Maybe they simply fail to supply the amplifier with enough current to withstand "heavy" passages of music, I really don't know.
Not three sets of headphones, 5 sets, Sennheiser, AKG, Philips, Samsung and Sony.
But let us wait until I will clean the card and try to do the best I can in replacing ribbon cable, moving to another slot, whatever I can and report the results. It is clear as it stands, nothing can be done without a direct action to sound card. I would have LOVED to have (and/or to know how to use it) a software program that could detect imperfections in DACs or analogue amplifiers. Something like generating some predefined sounds and prompting me to watch some sine waves on a software oscilloscope and notice if the visual waves stay within normal limits. I am fearful the card may have a voltage leakage somewhere that mixes with the "correct" sound.