Elberoth,
One of the best combinations I have heard with the Avalon Diamonds is using Jeff Rowland Design Group amps. Neil Lucent and Jeff Rowland are good friends and Neil uses Jeff Rowland amps to power his speakers. A little known fact is Jeff Rowland actually started Avalon in the mid 90's and sold the company to focus on his amps and preamps.
I would suggest the 302's either in stereo or mono depending on your budjet. I live in Colorado Springs and if I can help you with the purchase, assuming their are no dealers in your area, I would be happy to do so. I currently have Sonus Faber Creomona's but will be using them in a theatre setup after I get my Avalon Diamonds.
I built a room almost identical to yours in size and did many of the soundproofing techniques you used. All in all I am very satisfied with Jeff Rowlands build quality and sound. Thirty years from now after all my other equipment has failed or considered obsolete I will still be using my amps from Jeff.
Best regards and good luck with your project.
Colin