Three part answer:
1. I don't know.
2. Seems like the composition of the pad under the carpet wouldn't make any noticeable difference. It's under the carpet, plus either way, it's padding.
3. Refer to #1.

I had one room that had plaster instead of dry wall and it was one of the best rooms I've had a system in to date. All of the drywall in the room resonates at exactly the same frequency, adding a noticeable room bump. There is a good chance that most of your bass problems were due to the floor so you might have to re-tune the room after the new floor is in. The carpet pad wouldn't significantly impact the bass, the frequencies you are dealing with don't even "see" the pad. Anything that will make the floor more rigid will make the bass deeper and more articulate. I've known people that have used floor jacks, with great results. You should read "How to Get the Best Sound from Your Room", a chapter of Robert Harley's book "The Complete Guide to High-End Audio."

If your are really ambitious you could read "The Master Handbook of Acoustics" by F. Alton Everest. It's not that expensive but it is a very very tough read. Very dry and academic, but very rewarding.

Quote:
a person whose opinion i trust suggested using a fiber pad, not the typical foam and that it would help tame the bass quite a bit.

No carpet pad is anywhere near thick enough to have any noticeable affect at bass frequencies.

--Ethan