After the mods (KSU2 being a complete x-over rebuild and cabinet dampening w/Blackhole5), everything was different, and I think better. There was more speed and slam in the bottom end, playing quite deep with ease and authority, the "gap" between the ribbon and bass was virtually gone (I don't notice it anymore, but others may when comparing to another speaker), imaging sharpened up a bit, and the ribbon became more detailed and smoother.

I have two pair of 645s, one still stock, and I will definitely be modding them in the future (or playing with dig. x-over etc.)

As for the listening distance, at 7-8' there is still a small vertical sweet spot, which you can try to enhance with tilting the cabinet and tweeter, but I can't say for certain how effective that would be.

I think there is still a strong following for the speaker, just that NFR has some new designs available now, and we are all lurking in wait for someone to try them...

If you have never heard the ribbon, you can try to find someone in your area by posting on the Yahoo! Group "Newformgroup", many owners there offer to let others listen. I am on the west coast of Canada, on Vancouver Island, if someone nearby is curious.