what is the weakest link in my audio chain

Kimber Kable 8tc speaker cables, and a PS Audio Quintet power conditioner. I love the Paradigms, I'm sure the set up sounds great. Mix in a Project Debut III turntable with the built in phono-pre next, you will love it.

Power conditioning for at least the front end part of your system would probably shock you. Making improvements in a system is a crap shoot at best. Thats why this is a lifelong hobby.

I agree with Larry - try a power conditioner. Also for two channel you might consider a high quality jitter immune DAC but it will depend on how much jitter you have on the Toslink (it can work well with your AV8 or it can be terrible but it it is hard to predict). I think your speakers are excellent already but you might consider moving fronts to rear and going for the 100's at some point.

FWIW - You have an excellent setup already.