Jford, I had an SLI-80 that came with Tesla KT-88's which burned out and I replaced them with the Svetlana KT-88's which were a huge improvement in the sound, especially in the bass. Can't say what 6550's sound like but I was under the impression that the newer model of the SLI-80 came with them instead of the kt88's in the older model (which I owned), hence my question. I've since gone onto a 15w SET amp and still miss the tightness of the bass in the Cary. Kinda surprised that the bass is an issue for you.
I asked about the treatment because the large window. Figured a curtain might help. I too have a difficult room and have had some success with DIY bass traps ala Jon Risch and strategically placed bookshelves, the odd piece of foam and curtains. Not ideal, mostly because of the shape but I'm liking my sound too.
What have you compared your speakers too. I once saw a pair of the same Jamo's being sold as a demo pair locally and considered them but I couldn't get a listen or the money.