Want more high frequency extension

If you dont have some,try some silver speakers cables.

Construct a time machine and go back to your teens; preferably female.
Buy the Rives audio test-tone CD and run some upper end tests. You'll (probably) cry yourself to sleep for a week.
Check out conversations with your parents; that's what your hearing is going to turn into.
Get your ears cleaned professionaly once a year.
Unless your age starts with a 1 or maybe a 2, 15kHZ is probably a long distant memory. And 10k ain't that far off.
Most of the above advice is useless, unless you first determine what *your hearing is capable of, before what your *system is capable of.

Matty, the Green Mountain speakers are definitely NOT the problem. While I wouldn't call them bright, the word "engaging" comes to mind.

Also, zero feedback tube amp are more forward than those with feedback, in my experience. The differences are not even close. Right now, my current amps allow one to dial in increasing amounts of feedback, and adding feedback only serves to destroy the sound by taking away extension at both frequency ends and removing all of the music's excitement.

Tube amps with no feedback are also, in my experience, those most reflective of the sonic signatures of their tubes. By applying feedback, you get away from that. In your case, you can use it to your benefit.

Your preamp is known for being on the exciting side, so I would still first change out the Bugle Boys for something more extended. Try Siemens 6922, as they are on the opposite end of the spectrum from the Bugle Boys.

After that, I would roll the tubes in the SixPacs, beginning with the EL84, then moving to the output tubes. One thing I would worry about in going to tubes other than EL34 is the issue many have faced in doing the same thing with the Cary V12. Surprisingly, many here who have tried KT88 have noticed less excitement, jump, power, and bass when moving from the EL34. Despite their speaking to Cary regarding the bias settings, I am 100% sure that the bias settings for these larger tubes is set too low based on the sound.

One EL34 flavor which is more bright than you have now are the JJ E34L tubes. I have a quad of blue E34L coming today, so I maybe able to give you a report on how they compare to my JJ KT88.

But, and this requires a big leap of faith, and the ability to do it, the biggest improvement I have been able to wrest from my system this year was REMOVING THE PREAMP altogether! Running CD direct, straight into my tube power amps increased the jump factor more than I can ever convey. It is one of the biggest improvements I have EVER seen. I was running a Blue Circle BC3, and removing it ratcheted up the excitement, immediacy, slam, bass, power, everything. In later trying a Joule LA150, which was clearly superior to the Blue Circle, the sound was still no match whatsoever for having no preamp there.

The question is, can you do this? Do you spin vinyl? Do you run more than one source? Do you have a CD player with variable out? Are you willing to try one? What about using something like the EVS Ultimate Attenuators? While it is simple, it isn't for everyone...

Trelja...While I cannot disagree with your preference for low (or zero) feedback amps, I have difficulty believing that the difference you hear derives from greater bandwidth. Reduction of feedback will change the sound so that it reflects the characteristics of the amplifier, (most likely the output transformer) rather than the input signal. I would expect the bandwidth to be less with low feedback, and this may emphasize harmonics in the 12 to 18 KHz range that you may find pleasing.
The ability to vary the feedback is interesting. Do you have, or can you generate frequency response plots for various degrees of feedback?

Also, is the amp really "zero" feedback? No local feedback around stages, or "ultralinear" transformer windings driving output tube grids?

A schematic would be nice.

Mattybumpkin...A word of encouragement!

You, like eveyone else, have lost, or will lose high frequency hearing. When tests are run, (at least on me) sine waves of various frequencies are used. I cannot hear a sine wave much over 10 or 12 KHz, but I can clearly detect the action of a supertweeter or a filter operating up around 20KHz. My theory to explain this is that the ear senses rate of change of pressure as well as pressure change, and the steep wavefront of the HF sound is sensed, even if the frequency is not. Hey..it works for me.

Also, loss of HF hearing does not, at least for me, affect spatial perception, eg: imaging.