I apologize if this has already been discussed, but I couldn't find a post that addressed this.

I used to have an Onkyo sub (100 model) that was part of a Home Theater in a Box. I replaced it with a Klipsch Sub10 on after thanksgiving day sale at Bestbuy. It was on sale, had 10" woofer and more power the Onkyo.

The Onkyo had a block of wood on the bottom. The Klipsch has 'feet', but no block of wood underneath it. Would adding a 2x4 or placing a block of wood under the sub have any benefits over keeping the sub on its feet on carpet? Out of curiosity, does anyone know why Onkyo has a wood piece on the bottom of the sub? Does this help reflect bass?

Thanks!

I assume by block of wood you mean the bottom plate provided on some downfiring subs.
Unfortunately it can go either way -- some folks find adding such a board helps over carpet and some don't. Some find adding adjustable spikes to transfer the sub's weight directly to the floor help, and some don't.

Two things for sure:
The quickest way to find out on the board is to try it, and
If your carpet/pad is thick enough you can see or feel the sub moving up and down, you are wasting bass energy. Use spikes or add weight on top of the sub.

In case it comes up I'll add the how much space do you need under the sub calc.
This is an "optimal minimum" distance based on the assumption you don't want to have to compress the air any more than necessary -- downfiring subs will work with less distance than this between driver and plate/carpet/floor.

Determine the working area of the cone by measuring accross the driver from one peak of the surround to the other. For a 10" driver let's say this measures 8".
Calculate the area, pi x 4 squared = about 50 square inches. So you want to make sure the air does not have to squeeze through an area smaller than 50 square inches under the sub.

On a lot of downfiring subs the peak of the surround is the part closest to the plate/carpet/floor. That was a circle 8" in diameter. 50 / (8 x pi) = about 2".
So the peak of the surround should be no closer than 2" from the carpet. But we need to take into account that under peak excursion the surround may move closer to the floor, so lets make it 2 1/2" just to be sure.

If you have less than this amount of space between your driver and the carpet you might gain slightly by increasing it if you can.