Proper method when rack is not on cement floor

No problem. Is the idea to "push up" a bit on the jack once it has been put into place? That is, to put some counterpressure on what is probably a sagging floor? If so, how is that done with these jacks?

Yes, that is the idea. The jacks are very heavy steal, with plates at top and bottom, and a threaded rod at one end. They telescope to several interim lengths, you choose the length that is just short of your overall height from floor-to-joist. Then you turn the threaded rod until tight between floor and joist, and turn a bit more to either add just a touch of support or a bit more beyond that to lift out a sag. By using a number of these, you can end up with a very rigid, very strong, floor.
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Okay. Just have to find someone willing to get into that crawlspace! I'm too big a guy to squeeze into something like that. I took another look at it, and it's more along the lines of two feet or so.

Tony, Rushton's idea is a good one, but let me offer one more bit of advice to take that a step further. When installing the floor jacks, mount a 4' x 4' on top of them and use three jacks. The jacks themselves should be spread out but spaced so that they are directly underneath floor joists. Even though the 4x4 will lift and support the flooring, it is better to have the jacks directly underneath the joists for what should be obvious reasons.

By using the 4x4, you lift abd support that whole section of the floor in a somewhat even manner, making it even more rigid. Placing large sections of flat patio type stone under the floor jacks also spreads the load out over a wider area and keeps the jacks themselves from sinking into the soil.

This whole project shouldn't cost you more than about $30 - $40 at the very most and should make a substantial difference in the solidity of your flooring and what you hear. I just hope you've got a good friend that is both skinny and limber that works cheap : ) Sean
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That's a great suggestion, Sean. Thanks. I can easily get a 4x4 cut to the right size at my local lumberyard. But, I have some questions...

The room where the equipment will be placed is 11' deep x 17' wide. The floor joists run from front to back, parallel to the 11' depth. So should I use just one 4x4 stringer that's about 16' long, set about halfway back into the room? The room is actually the back extension of my media / family room; the front part is about 12 x 17, and is over a full basement. That's where the media display and the front speakers are located. Yes, I understand that's counterintuitive. I should brace the flooring under the crawlspace and put the speakers and display over there. Tried it, didn't like it. It also did not meet the WAF. So I have to do what I'm doing. Given that my speakers are actually in a different part of this combined space, how much improvement will be secured if I do the bracing, but really have only the equipment in this area?