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?

Best,
Mihalis