After having the SET 400 and Alta Audio Titanium Hestia's for a few weeks I can truly say they are a synergistic match. The sound fills a large living area with ease and authority. The imaging is uncanny and bass is full with out ever getting in the way. I have tried the 400R with the Alta's and there are more things that are similar than different. The 400R has a little deeper soundstage with a slightly liquid sound compared to the SET 400. The Set 400 is more forward with the air of the recording well into the room with you. I could live with either and be happy for a long time. For speakers that need some balancing in frequency response, either because they are built like that or room interactions, I would go with the 400R as tube rolling allows you to dial in your sound. However, the Hestia's are so well balanced that the SET 400 is perfect for them. The SET amp definitely has a unique sound that should be heard by any of you that think a fully transistor amp can't convey the emotion of the music. A $2000 amp running $32000 speakers, blasphemy, I know, but I don't think I could do much better for 5 times the price. The guy I bought the Hestia's from ran them on the SET 600 monoblocs and said they sounded better than the SET 400. I might consider trading up one day but I have plenty of power now and am enjoying the combination