Best Upgrade?

Here is an interesting question. My system presently includes:
EMM DAC6e/Philips 1000
Hovland HP100
BAT 75se
Alto Utopia Be
Synergistic Resolution Reference cables
PS Audio Ultimate Outlet

I am thinking of the next upgrade but obviously can not hear components at home. Either change the HP100, or add a 75se to turn it mono at 150Ws, or get better power conditioning?

How does one really make a good decision given all the variables?

Thank you all. I think I will focus on room accoustics first. Changes in speaker positioning can be heard loud and clear and my room is a little bright given that it is full of glass windows. Interesting though that there is no consensus!
Very best. 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???
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.