I am sort of amazed that no one other than Geared 4me has suggested listening angle could be the cause. The FIRST STEP in any speaker set up is being sure you are sitting within the speaker/tweeter "dispersion pattern". For example, if you are sitting above the tweeter, you may be out of the pattern won’t be hearing it correctly- usually much less top end. If your stands are too short, or your floor standers are too low compared to your couch, or your speakers get very narrow on vertical dispersion and you are slightly out of that pattern, your speakers will sound dark every time. Buying cables to make it brighter is fixing problem you may not have and just gets you further away from the real problem.
Look at your speaker’s vertical dispersion, which is typically 10-15 degrees in "height". This means that the speaker only sounds correct within this 10-15 degree area of vertical dispersion. A big generality, but is usually about right, is a tweeter typically shoots straight out on the upper angle (parallel to the floor) and the lower angle points down 10 to 15 degrees. Imagine your tweeter as a flashlight with a 15 degree pattern of light- its the same as that. This means you have to be within this area to hear it correctly as designed. So this is why we say your ear should line up with the tweeter or the speaker should be just slightly above your ear.
If you listen above this dispersion area, the tweeter output WILL be very low (by design) and the highs will appear to bad, low or even non existent. You can check what you situation is by just lowering yourself to be within that vertical dispersion angle and see if it gets better. If changing your height relative to the speaker makes it sound better, gives you more high end, then being at the wrong angle to the speaker is your entire problem. If it doesn’t get better with vertical changes in listening position, you have a different problem.
You can repeat this process with the horizontal plane as well, sit directly on center of one speaker at a time (turn off the other speaker so you can concentrate) L or R and then move slowly off that axis and see how the high end changes, and believe me-it does. Many speakers have poor vertical and or poor horizontal dispersion and require you be seated directly "on axis" (meaning pointed straight at your ears both vertically and horizontally) to hear it correctly. Sometimes the correct area is so small that just a tiny move of your head changes the speaker's sound drastically. This on axis listening position is what is called the "sweet spot" and can be small or large depending on many factors, sometimes having nothing to do with the speaker itself. It could also be your room and how reflective it is.
There are speakers that are better off axis and some that are terrible off axis. This "good off axis" is one of the features of a "better" loudspeaker that might be unnoticed on your first demo.
Brad