what is the weakest link in my audio chain

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.

Thanks for the valuable advice. looked like the next upgrade that can gives noticeable sonic improvement with minimal investment would be to use better speaker wires.

FWIW: 10 AWG is more than thick enough already. I doubt you can eek much more out of speaker cables unless you can shorten them. Room Acoustic treatment would be much more productive.