Do "Cardas Caps" make a difference?

Yes. On my Audiomat Arpege they 'darken' the sonics to a point that I find quite objectionable. Proceed with caution!

I just installed a new pack of 12 on my KRC-HR and KSA-300S. I know when using my theater thruput because I have a long run of RCA's I used to have a slight low level buzz. Now there is nothing and the system seems even quieter. I only put them on my reference auidio (not video), and to me they can only help IMHO. Sean has great advice too, but I opted for these instead.

I made up shorted RCA's (guess these are shunts) for the unused inputs on my Radio Shack battery powered phono preamps (two used as dual mono's, thus unused inputs/outputs on each unit) and soldered 100 ohm resistors across RCA terminals and placed them on the unused output's of the preamps. The results: even quiter phono stages (these battery powered units were very quiet to begin with) and just better overall sound. As far as the Cardas Caps go, I have never tried them and instead used rounds of tin foil (on unused inputs) in the past. There was no difference in the sound, that I could hear, but I figured that they would help block dust (especially on my Musical Fidelity amp which is not a lot of fun to open up). I don't bother with them on "open" tube gear.