Is Kimber 8TC a significant upgrade from 8PR?

The VTL amps do not have very good specs. High distortion. Moving to a relatively distortion free amp, say, THD and IMD in the vicnity of <0.005%, instead of the 1-2% of so many so called high-end products, will be significant. Where cables, top end components, and tweaks, are of questionable value at best.

Are you into dipole speakers, say, electrostatic, or planer? There is a myth that because they tend to be revealing that a 'warming' (=distortion) tube amp is needed.

Truth be told, what they reveal is the sonic quality of the source material which is all to commonly bright, and otherwise inferior. Or other distoriton producing components. The system gets blamed for its very superior quality.

Ironic.

Then one is traveling down the ludicrise path of using the system to mix out inferior qualities of all too many recordings, which at the same time is masking any sonic superiority of other material.

Best bet is watch e-bay for some teflon insulated 18 guauge silver plate and quad them, terminate them and spend the balance on music. About as good as it gets for the equipment you will pair it with.

To answer your question as directly as possible, the move from 8PR to 8TC may produce some subtle audible improvement, but don't expect anything dramatic. The main difference between these two cables is the dielectric (insulation), with the 8TC being coated with Teflon. I have owned 4PR, 8VS, 4TC, and 8TC, and can't reliably hear differences between them. (However, that may be due in large part of my aging ears, since I don't hear tones above 10kHz.)

If you want to upgrade your speaker cables, I recommend you do what I did when I upgraded from 8TC: buy Alpha-Core Goertz MI2 cables. There was a long thread here on Audiogon several months ago, with some excellent comments by our resident guru, Sean. Look in the archives for the following thread:
http://forum.audiogon.com/cgi-bin/fr.pl?fcabl&1093784344&read&keyw&zzgoertz

The Alpha-Core Goertz MI2 sells for about the same price as the 8TC, but in my opinion is a superior speaker cable. For more info on this cable, go to Alpha-Core's informative web site:
http://www.alphacore.com/goertz.htm

Just auditioned the 8TC and am sending them back. They have a very tinny sound. Having a dis pleasingly thin, metallic sound. Going back to the 8VS which is much warmer and natural.