Best Upgrade?

...my room is (...) is full of glass windows.

It sounds like there's not a whole "damnation" of a lot you can do about room treatment (heavy velvet curtains @ 1st reflection points??? Even that might only work over ~5kHz or so).

Rather than strain the purse with expensive h/ware upgrades (at yr level, "better" would mean giga$), why not play with the following cheaper "passe-temps":
-DIY power cables
-A transformer coupled volume control -- disconnect the pre(diyhifisupply in HK has a good one)
-Try power amp plugged directly into wall socket (then, pre+power)

If you DO consider there is significantly more to be had from yr (excellent) system than you're getting, I would try a different pair of (94+db sensitive) spkrs. Obviously not at home, as you note -- but @ a dealer???
Cheers

I just added a BPT 3.5 Signature to my system. I do have a 20 amp dedicated line with hospital grade plugs. It is without doubt, the most incredible update that I have EVER done. I am just stunned at the difference in the sound. The suggestion above, that you begin by building a solid foundation, is dead on target. I have been upgradeing for about 8 years and not one upgrade made as big an improvement as this power conditioner.

In this order, I agree with Mihalis that the room and equiptment placement would make the most noticeable difference. Secound the power conditioner along with good power cords. They don't have to cost a million bucks, but maybe a couple of hundred. The difference I say was amazing. I would recommended this before upgrading your components.