I think you are barking up the wrong tree by looking at cables. The Straightwire Quartet is already a fairly soft sounding cable. Room treatment sounds like it is the issue, but seems to be out of the question - you need to work harder on training your wife. Try letting her know how much you love good music - after all her love for you should overcome the odd rug??? The alternative is to rearrange things so that you can listen in the near-field (say within 9 feet) with you and the speakers well away from the walls - even if this is not practical given the wife factor, you can still just try it one afternoon to understand what your system would sound like without nasty room reflections. Your comment about rugs indicates you have a hard floor. This leads me to suspect that the problem may be "flutter echo" which is caused by two parallel hard surfaces (ie. your floor and ceiling). You cannot fix this with cables or components. If you cannot treat the room, the only solution is to listen in the nearfield and preferably get speakers that do not have high vertical dispersion (planars or perhaps Spendors). But other things to tame a bright system are: Power filtering and good AC cords (for a cheap soft sounding one try the Black or Purple XLO cords); Using Vibrapods under your electronics definitely recesses the upper mids (but try the different grades to get the best result); and a real overkill method of getting smooth sound is Vibrapods under the speakers. I have also found some of the wooden racks can cause brightness - so try a proper rack (not with glass shelves) if you have not already.
