I like the Duo a lot. It IS somewhat difficult to set up and get the bass to integrate well with the horns. I don't think one ever can get great integration, but, there is no speaker system that is perfect in all respects. While I do think the Duo would work well with a wide range of amps, they are worthy of the best. There are plenty of good suggestions made above. Everyone has their own taste, so you need to figure that part out for yourself. In SET amps, I like 2a3 and 45 tubes myself. Although power is not really an issue, I think you should also explore some lower-powered pushpull amps. I like many 6L6 and KT 66 pushpull amps myself.
While the Duo is extremely dynamic and lively so they don't need "help" in that aspect of their performance, I have heard them with OTL amps and they sounded quite amazing with such amps. There aren't too many commercial OTL amps out there, but perhaps something from Atmasphere would be worthy of consideration. I've heard a few of their amps that were a bit noisy with very high efficiency speakers, but, they are still good candidates.