Upgrading from Primaluna

2 ft from the side walls sound correct. However, that measurement should be from the tweeter or center of the speaker...not taken from the outside edge of the speaker. 

And yes, do add diffusion/absorption on the front wall and look at the ceiling as well. These will make the speakers 'see' your room as bigger.

Think of the front 1/3 of the space as the highest priority to 'soften' and diffuse.

I had my Evo 400 running my old 88db 6ohm Salk speakers on the 4ohm taps for years before I tried the 8ohm taps for kicks. Upper treble and mids were much better with 8ohm taps with no adverse response from the bass. I could have kicked myself. I assumed because the Salks dip to 3ohms that the 4ohm taps were the way to go.

If you run the numbers, the DPHP should have enough power to push your speakers to ear damaging volume cleanly. I used mine with 87dB Dynaudios until I moved on to more efficient speakers. Having options is good though. A solid state amp could be a nice complement to the DPHP, even if it’s more of a lateral move than a clear upgrade. Sorry I don’t have any recommendations though. I went for low power Class A which wouldn’t have enough grunt for your situation. 

I had a HP in a smaller room with power needy speakers and went to a more powerful integrated. For my setup it was just the ticket. More punch and a fuller sound at low volumes too. KT120/150’s would be cheaper than other options despite current tube costs. 
 

Toe in those bad boys. 

I would advise a consultation with Jeff at HD Acoustics. He specializes in small room acoustics. He designed my new listening room and it would be well worth it to have him take a look at your room. Good luck!

Ron