D'Agostino vs. Ayre

Agree with khrys on tube choices but will note that the ARC Ref 250se works well with Alexias as do VTL S400. More power is better but there is a cost-benefit analysis to be done.
I've only heard DAG monos on Alexias but the Dag s 250s were in house with my Sashas and I felt that they didn't break a sweat. Call Dan at the shop and ask him what the difference will be. I'm sure he will be straightforward in his response. 

Can't speak to the sound, but if you're considering spending this kinda coin, you should also be concerned about support and service.  I have an Ayre product, and when I called them, they coldly told me that they had dealers to deal with questions from their product owners.  Never again will I buy an Ayre product.  Life is just too short to deal with arrogant dik-heads.

Your experience is contrary to mine. I've found Ayre customer service, Michael especially, to deliver exemplary service and support. I've owned Ayre back since the K-1, V-1 days. They have done several upgrades for me, given tech support over the phone, even provided a resistor change to increase the gain of my KX-R Twenty preamp. I'm Much more than satisfied with Ayre factory support.

ingoodwithtools      Me too

@aardvarkbark  You are the only person I've heard ever make that complaint.  Ayre's service is second to none. I've called them many times and they've always been helfpful (and honest even when it's an answer they know I'm not going to want to hear).  In fact, I would say if you value service and a reasonably priced upgrade path for your expensive components, Ayre is the gold standard of the industry in that regard.