Very pertinent points you raise, Eldartford.
While the Granite schematic is available to me, I have not asked for it, and do not have it. From their advertising, the amp is zero feedback, based on that setting. A switch controls whether the amp runs in triode or ultralinear (pentode), which I keep in triode with the KT88 tubes. And, while I have not measured the sound using feedback versus no feedback, it is instantly apparent there are dramatic differences between the two. Others who have come to listen cannot believe the degree. I could plot a response curve using a test CD, but in all honesty (and not to be a wiseacre), I always prefer listening to music as opposed to test tones.
I also concur that loss of HF hearing should not be used in any way, or at any time, as a barometer of one's ability to judge a system. Using a test tone (yes, I do use them at times), on my Fried speakers, I was able to hear the 16 kHz tone, but not the 20 kHz. I didn't much care, as I said, I prefer listening to music (which I probably the reason I didn't then go try the 20 kHz tone on my Coincidents), but the Frieds are only rated to 18 kHz, so I am now unsure if my inability to hear the 20 kHz tone was a reflection of the speakers or my own hearing. I do know most men cannot hear to 20 kHz.
Mattybumpkin, I gave the JJ E34L tubes a go last night. They were much less sparkly than the JJ KT88, so that should be a real consideration in your quest. Whereas the KT88 have a beautiful, sweet, extended signature, the E34L were much more flat in tone. It was akin to adding feedback, but not to the same degree in terms of magnitude. The KT88 were obviously much more powerful and extended at both ends of the spectrum.
I then leveled the playing field a bit by then running the E34L in ultralinear. While the sonic signatures of each tube remained, the power differences were more or less canceled. The E34L produced better bass than I expected, but the KT88 were still superior.
Based on what I heard last night, and what you are looking for, the KT88 are the more than obvious choice. Still, as you pointed out, you will have to wade into these waters VERY carefully, to avoid the issues others have faced in running this tube in the SixPacs. Due to the cost of the larger tube, it is not something to get into without being certain of the result.
Good luck!
