You could try either. My previous table (also suspended btw) had a severe FF issue. I used floor jacks, and it solved the issue 100%. In the adjacent room, I set up a second system, and used a wall shelf on an interior wall. It worked good, but there were times when closing a cabinet/drawer on common wall caused a FF. I have since removed the shelf, and am temporarily an using an Ikea rack. No issues thus far, but I just made the move.
I just traded an LP12 in for a RP10 and had severe FF issues with my floor.
I had two Lally columns in my basement supporting three joists to eliminate
the flex in the floor. I also had a brace between the wall and the top of my
cabinet to eliminate any rocking movement at the top of the cabinet. If I
didn't step softly I could still force a skip. With the same cabinet and all
bracing removed, I have yet to have the RP10 skip.
My suggestion is get the table and see how it reacts first. Then I would look
at a wall shelf before I would ever brace the floor again. Soil can expand
and that expansion could cause the jacks/supports to force the floor joists
up and possibly do drywall damage or damage to the floor joists. That was
one of my primary reasons for trading off the suspended table, I had
noticed movement in the structure of the house that concerned me, e.g. a
door a few feet away not closing. I love the RP10 but have to admit now
after a time that the LP12 had a better sound that I miss. But it is close
enough and eliminates the structural concerns of a larger investment.
That said the shelf makes much more sense in the long term picture. Or at
least that is my opinion.
Don't get Mike Holmes started on that one...
Really though, if FF is so bad it is most likely going to muck up your bass as well.
A decent carpenter should be able to take a lot of the bounce out of the floor without much effort. You could either "toenail" in extra boards to the joists to make the stronger with less flex and/or put cross members between the joists. Either way, you would need a nail gun, a bunch of wood glue and some inexpensive 2x6 or 2x8 boards anywhere from 4 feet to 12 feet long...
it depends on how technical you want to get. Your interior walls that you may want to hang your table could be supported by the bouncy floors you mentioned. An exterior wall would load down to the foaundation in most cases and be the best solution for wall mount. The simple way to take some bounce out of a floor is to add a row or rows of bridging perpendicular to and flush with the floor joists. You are looking to increase your "load bearing" of your floor system and a google search of that would guide you further down the path of obcession.
I put a Vibraplane isolation platform under my suspended table and it solved all of the problems you mention but also isolates the table so well that I hear all sorts of new detail and extension. The sound improved tremendously. Details can be found on my system page.
My effective inexpensive solution for a suspended turntable (B&O) was to place cryogenically treated tempered steel isolation springs (Machina Dynamica Cryo Baby Promethean Mini-isolators) between the maple board the turntable rests upon and the horizontal top furniture shelf beneath.
For further vibration control, a VibraBlock Damper rests on the horizontal top furniture shelf.
As mentioned by Peterayer above, the sound is upgraded to a surprisingly impressive degree with appropriate isolation.
My turntable is protected down to about 3 Hz which is incredible for the low cost of the setup. When Godzilla next stalks the earth my vinyl won't miss a beat.
My living room has a suspended wood floor with 20' spans for L and W. A closet, enclosed stairwell and fireplace column on one wall prevent the dreaded "square room" for sonics. But the long span meant bouncy motion even when walking light-footed. I had a heavy cabinet placed by a sidewall but the suspended table still bounced the arm/stylus whenever anyone moved around the room.
I too have a crawl space so I installed three 4x4 posts on concrete footers at 5', 10', and 15' along the center axis of the room. My floor joists run perpendicular to the line of the posts so that helped distribute the support. That cured my floor flex and bouncing stylus problem.
An easier solution might be to mount your turntable support on a load bearing wall. But my floor supports also minimized the floor flex to improve bass response (stiffer boundary) so I gained two benefits.
If you have much flex to your floor it would seem to me any sort of suspension under your table would only be a band aid. Why not cure the ailment instead of taking an aspirin?
I should have explained that I used an automotive hydraulic jack to raise each of the three floor joists just enough to slip the carefully measured 4x4 posts into place. Then after lowering the jack I didn't even need to nail the 4x4 posts to the joists, but I did drive a couple of framing nails just in case.
Also heavy duty leveling jacks are available for mobile homes, but the three posts and footers were not expensive.
For an easy fix, cram a few paperback books behind the rack or furniture the 'table sits on (as close to the top of the rack as possible) so that the wall will essentially brace the rack and keep it from moving too much as the floor bounces. The more tightly you can wedge the books in there, the better. Fremer mentioned this somewhere on Analog Planet, and it works.
Sounds like you have the right idea and bolstering up the floor in your case by jacking it up from beneath would be easy to do.
I think a wall shelf would be best unless you have cabinets or something else on that wall that would themselves cause vibrations when accessed. But jacking up the floor beneath that wall will make that wall even more stable. And if you decided against a wall shelf a platform/table close to the wall also minimizes foot induced vibration.
Rega recommends against heavy dense platforms for their turntable design to sit on. I asked about making my own concrete slab for stability and they discouraged that method, saying that it tends to store and then amplify energy/vibrations. Suggesting instead a lighter thinner surface much like the Rega shelves. One that in their words dissipates the energy instead, much like their lightweight turntable designs are intended to do.
Ablang.....yes - very effective.
Not sure is this is relevant, but I bought an older but clean Linn table a few years back and stuck on top of my rack where it was NOT happy...then (of course) read the "official" manual which suggested a lower firm table and it's worked perfectly since. Low mass, level, sturdy table...done. Got lucky I suppose.