Paul, I actually ran an ARC Ref5se for a while with a pair of 275's. It was a big improvement over the Mc 2300. I was never impressed with the 2300 as the sound was rather sluggish and muddy.
The Ref5se cleaned it up considerably. Sound was much more coherent and dynamic. The Ref5se is an excellent preamp, one of the best in its price range
That said after listening to a Shindo preamp I was completely blown away. So very musical!
As soon as I got delivery of my Shindo stuff the ARC and Mc gear were sold, never to return.
Not trying to stir the pot even more but when you decide to change your digital source to a really good DAC all the upgrades you are making will sound dramatically better. The difference in sound quality will be HUGE!
I'm so glad you are seeing the way with efficient speakers. The fluidity with which the music comes through, effortless and unforced is truly magic. My next trade up will be even more efficient 16 ohm Shindo or Auditorium 23 speakers. The moment I have a dedicated room like yours they will be on order.