I liked the look of the KH310's. However, I decided that I wanted to try surround and maybe multi-channel later on. Buying a dozen KH310's seemed a bit expensive to me so I settled for the KH120's. Also, Neumann say that any KH120 will be well-matched to any other KH120. So spreading the purchase over a few years may work out. Whether I need another before they go out of production is another matter.
The KH120's have less bass than the, much cheaper, speakers they replaced in my set up but they are much smaller. I figured that as I was going to get a sub anyway it may as well work for a living. The other point was that the rear speakers would be placed on a bookshelf and the KH120's would fit. Even the port faced forwards and all the speakers would match.
Subs from people like Neumann and Genelec are very expensive because they act as hubs for the complete system. I am sure they are good but I could not quite see the value for money aspect so I opted for something else.
My room is about as long as yours but less wide. It has a carpet and is lined with books and sheet music so it already has acoustic treatment.
The main problem is the bass. You can't live without it but you can't live with it, either. The piano is stuffed with material to hold certain panels in place and even door knobs have slips of cardboard judiciously inserted.
I have been tempted by something like this to help with my sub.
http://www.bkelec.com/HiFi/Sub_Woofers/ ... de--DC.htm It should limit the resonances while allowing a reasonable base level setting. Passive treatment for bass resonances is difficult. A lining of rock wool will not cut it. Basically, the size of the treatment and the sound wavelength are related.