I purchased a replacement muffler from the internet for my 2004 Xterra. It arrived yesterday but had some noticeable dents in the resonator. My question is: will these dings degrade the sound suppression of the resonator?
This is a very subjective question in what do you mean by noticeable? It also depends on the size of the resonator itself. The general answer would be, no, it won't. Most mufflers have fiberglass matting in them which absorbs the sound which flows into the muffler. The dents usually won't affect this. The only time it would is if the muffler is completely crushed in some way. If you had bad shipping or believe it came from the source that way, send it back for a replacement.