Swapped the 2x GE 6SN7GTB for 2x (Mil-Spec) CBS 6SN7GT. Soundstage on the K501 is now incredibly large and 3-dimensional. Ana Caram's voice and the music flow uninterruptedly--seamlessly--and, as it were, "unexpurgatedly." The CBS tubes render the transient detail more lively and crisp, as well as greatly enhance the perceived height, width and depth of the soundstage. These "easy" Brazilian jazz tracks play peerlessly through the K501.
Wow! How could I have missed all this my first time around w/the K501(!)? It could have been the source/amp (back then I had the RAM-modded Samsung HD-841/G&W T2.6F & Darkvoice 336i) or, more likely, I just didn't give these fine cans the time and attention they justly deserve(d).
In all fairness to the K701, they too sound exceptional: more finely detailed, warmer, more pleasantly rounded and tonally shaded; however, even given that list of rightfully deserved accolades, Caram's vocal just doesn't sound as believable [i.e. with the K701]. It sounds more forward/surfacene and/or unnaturally delineated/disjointed between its properly nasal and properly guttural characteristics (just as I mentioned earlier).
This disconnect in the vocal content of the track (w/the nasal aspect supplanting or superceding the guttural aspect) in turn propogates, more generally, into a reduced overall synergy between vocal and instrumental content of the song. As a result, the listening experience becomes more analytic than holistic. Mind you, it's a subtle distinction at first notice, but with further critical listening it becomes the concrete basis for discerning the fundamental difference in listening experience(s) between the K501 and the K701.
Well, does it mean that I'll forsake the K701? No. But neither will I let go of the K501, nor neglect their signal strength: a seamless and well-integrated midrange, particularly appreciable while listening to vocals. When the musical genre especially favors them (e.g. acoustic jazz, small vocal ensembles, chamber music, etc.), the K501 will be my "go-to" AKG 'phones; everything else--and that's actually quite a bit for those of us with ecclectic listening tastes--will be heard and enjoyed through the K701. : )