30 ft rca run too long for a phono to preamp?

Sorry to highjack my own thread, but this is turning into a turntable isolation thread.
Tvad, I am having a hard time finding info on ceiling hanging, is this better then a wall shelf, would I use heavy chain, and a slab of granite? poured concrete? I have moved the speaker and added 2 more isolation platforms and now I can stomp my foot close to the turntable without skipping, most of the time. I recently blew my 10" acoustic coupler voice coils on my Vandersteen 3a's, so the deep bass is not there right now, but I think it goes to about 35hz, and I am not getting any terribly noticeable feedback, but I think I could do better. I used the hi-fi new lp and my cartridge resonates at around 12 hz, so even with the couplers back in after repair, the speakers shouldn't cause an issue at 20or 25 hz. It would be nice to be able to completely isolate from the floor, and the wall is not bearing, and is underneath a 10ft window. It shakes when I bang on it.

Resonant frequency of the arm/cart has nothing to do with acoustic feedback, so don't let that fool you. Anything from about 200Hz down can cause issues.

You definitely have your hands full with springy floors and a somewhat tight space. I'm not sure how the Vandy's will react, but pulling them out from the back wall another foot might help with the pressure that is building up along that wall. And, if possible, it might be better to pull the table out from the wall as well.

Can you brace the floor from underneath?

I designed a shelf for my table, and emailed it to a local welder, then today while awaiting a reply, I decided to make one myself, just to make sure it would work. It did. I used 2" Mahogany, 4 1/2" stainless screws, glued, clamped and screwed. It's not fine furniture, but it is solid, goes with the equipment stand and room, and best of all it was free as I had all the materials on hand. I can jump up and down right in front of the turntable and it does not skip. My son and I tested it out well.
I tried the Hi-fi news test lp again, and It tracked the torture track with only the slightest distortion from the right channel, best yet. I moved the speakers into the room a bit, and while I don't have my super deep bass right now, I can play as loud as I want without audible feedback distortion. Depending on what the quote from the welder is, I may just keep my version and spend the rest on records. I am now looking to put a slab of granite, Mango wood, or a poured concrete slab ( like a countertop) and probably a sandbox, with the galibier metal inserts into the sand, and I think I'll be done for a week or so.

Thanks for all your help.

Mike, is your suspension on the tt working properly or is it getting hung up? I seem to remember the 147 as a sprung mount suspension.