Dougs951

I have dual 8" subs in my spare tire well and it sounds great imo

Cyberpunky

Really ? I would say the opposite, in that you would be hard pressed to hear even the slightest difference. Sub bass is such a low frequency it is very hard to suppress. That's why you can hear some cars coming from 2 blocks away.
I wouldn't put pillows over the sub, but they will push the sound right through carpet no probs. If you can have them in your boot/trunk going through metal and seats, then carpet just isn't an issue, and I have done a few stealth systems with subs firing through carpet. I would agree with putting carpet over full range speakers(midbass and up) but not subs
peace
Cyberpunky

Butters944

My friend's fiero has the sub under the front hood. It sounds great, but creates a nasty rattling in the front of the car

So I'd say no it doesn't affect much