I would like to achieve an increase in volume only in the highest end of the spectrum which we call Brilliance.
I find it works much better to look only for components that are really good. As a consequence of that its just never in 30 years been necessary to try and fix anything like what you're doing. Even if I did have a deficiency (which I do, everyone does, nothing ever is perfect) I would never go looking for something to patch it over. Because then I know its only a matter of time before something else has to go, and then I'm right back to grabbing another band-aid. To put over the problem. That I myself created.
If you can identify something in your system- power cord, interconnect, speaker cable, fuse, whatever- that is rolled off, then by all means upgrade that. If its the pre amp then look for a better one.
For sure its not your cart. AT are not rolled off. Quite the opposite. For sure its not your Bryston. Ditto. For sure its not the Herron. I have that. Its the best thing in your system. Definitely not that. Looking at reviews I seriously doubt its your pre-amp.
If you can't find the culprit then I would still not change the pre amp. That's about the last thing I would do. First would be Total Contact. For sure you will hear improved detail, sparkle, extension- but also tremendously improved imaging, depth, and harmonic structure that you will find hard to believe. Cable Elevators are another one to improve top end detail. Orange Fuse. Cables. Power conditioner. ECT, HFT. Long list of things I would do first.