I use Cardas Caps too. All the unused inputs on my pre have them. I threw one on the digital out of my cdp and didn't notice any sonic difference. I did it mostly to keep out dust.
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This is a very subtle tweak, but I beleive they offer an improvement. After covering all the unused jacks in my system with Cardas caps, I listened for several weeks. I then removed them all and noticed that the HF had a bit of an edge and the mids were a bit unfocused. I would recommend covering all the unused jacks, as this is a relatively inexpensive tweak.
As for unused XLRs, the same test yielded similar results, but the cost for Cardas XLR caps is several times the cost of RCA jacks.
Cardas caps are to prevent EMI/RFI intereference to enter the chassis thought the so called "darn little holes" in the middle of the RCA jacks. I used a total of 46 in my system and some equippment might not sound as good if you use some, and of cause some do. If I have to use them to cover ALL the RCAs I will br broke... do some blnd test to see if they help, then use... Also, they can be also as long as it's a RCA jack.