Do "Cardas Caps" make a difference?

I have a dozen and I frankly can't tell if there's an improvement in sound. Oh, well, they look good enough and keep the dust out. I won't buy any more of them though.

I have 51 scattered throughout my main system. I also have some Cardas XLR caps. (The female XLR caps don't fit; I'm waiting for an upgrade.) I haven't heard any difference. I think its a good idea to keep dust and mosquitos out of the electronics. I also feel that improvements can be cumulative, i.e., multiple tweaks that are not individually audible can add up to an audible improvement. Perhaps these caps would fit in that category.

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.