Hi Jtein,
I owned Avantgarde Duos for four years and have owned Duo Primos for the past four years. I must have owned and/or demoed a couple of dozen amplifiers with Avantgarde loudspeakers. Amplifer noise is the main issue with Avantgarde loudspeakers. Many amps are just too noisy to be used with Avantgarde loudspeakers. Tube amps that sounded good with Avantgarde's were: Cyrus Brenneman Cavalier Plus, Audiopax Model 88's, Art Audio PX-25, Wyetech Sapphires, and BAT VK-75SE. Solid-state amplifers that sounded good with my Avantgarde's were: Pass Aleph 3, Red Wine Audio Signature 30.2, First watt F3, Resolution Audio S-30, and Avantgarde Model 3 and Model 5+. I love the way tube amps sound, but can't stand the maintenance and reliability issues associated with them. I'm currently running a Great Northern Sound Reference modded Resolution Audio S-30 with my Duo Primos and it sounds fantastic! Good luck!