Broadstone concerning your query as to whether an Atma-sphere S30 is enough, I DO recall a thread regarding their use with the MM Decapo with some saying yeah others nay, not enough power. Given your room size I think you are on the fringe, it again would depend on your musical tastes and levels that you that you require. I do recall that Ralph mentioned that many of his customers use the S30 with Decapos which are essentially the same design and probably the same electrical characteristics or close at least. I would think 30 watts is pressing it and would go for the mono 60 watt amps if considering an OTL.
So far as the characteristic sound of your Cary vs a an OTL in general very different presentation. In my experience, I've owned 2 OTL amplifiers, that when mated with the right speaker they are certainly worthy of consideration, heat factors aside. The transients and rise time on an OTL are more like real music, very fast and dynamic. You are not going to get that added warmth that the Cary delivers. I feel OTLs can get closer to the sound of real music but it DOES take care in selection of matching ancillaries. The added transparency can work against you if this is not addressed properly. And yes, they can present the best of both worlds whereas cheaper SS designs have a tendency towards dryness and a 2 dimensional character there is a wide variation in the presentation of tube amps as well. So far as an OTL there is no substitute for listening for yourself. btw, the Atma-sphere amps I've listened to, always in unfamiliar systems, have just been superb in every way.