Well that idea turned out to be arse about face.
There are other factors that can affect bass quantity/quality, however the carpet isn't one of them. Your new room may be smaller, but does it have stud partion walls and/or a suspended floor? Does it have an open door/opening into another room?
Stud walls in particular can suck bass out a bit as, unlike a brick wall, they bend under pressure from the bass. This takes some of the energy out of the sound wave that would normally just be reflected by a brick wall.
Equally, an opening into another room can suck bass out at the frequencies specific to that rooms size, due to cancellations between it's modes and the listening rooms modes.
I still suspect the 'hardening of the sound is due to turning the sub up to compensate for some loss caused by the room(s). One idea might be to site the sub closer to your listening position. Simply, the closer it is, the louder it gets, the more you can turn it down.
Of course this might be just the excuse you need to uprgade your sub.