Having trouble reducing the vibrations on my turntable setup

Springs bounce and resonate. Townshend controls this with an ingenious air resistance damper. Without knowing what IsoAcoustics is doing it is hard to say. Probably a solid mass under the Podiums will be best.  But you will just have to try both ways to know for sure. 

Spikes unscrew. If they are the same thread size and pitch as Pods you luck out. Get a stud and screw the Pods on in place of the spikes. Pods are threaded to allow the top to be turned for height adjustment. Makes turntable leveling a breeze. You could use them that way. At 50 lbs your turntable will be stable on them. But you could also screw them together and still have the height adjustment. Whichever you prefer. Just pointing out options here.

Sorbothane, occasionally you find some place it does what you want. In general though it is so soft it sucks the life right out of the music. A much better material is fO.q tape. This used in the right places will make a huge improvement. But it works on micro- to nano-vibrations primarily midrange and up. The problems you are hearing are much larger amplitude/lower frequency. 

Ditch the spikes altogether. I tried a whole bunch of em, used BDR Cones for years. Pods beat them all. No agonizing is it really better, or its better this way but what about that? None of that. Not even close.

What is the  Delos butcher block sitting on? How stable is the stand it sits on?

I have a ClearAudio Innovation Compact with Universal arm. It sits on a VPI turntable stand filled with sand. On top of that is a butcher block that sits on cones. My turntable sits on the butcher block with no special footers. My setup is dead silent. No rumble or noise if you walk by the stand.

 The Innovation turntables have magnetic bearing that should isolate the platter. Where is this rumble coming from, the motor? There could also be some type of problem with the setup.

Any shelf will vibrate at a certain frequency.  Depending upon that frequency, there will be regularly spaced nodes that remain stationary even while the shelf is energized and vibrating.  If you can locate those stationary nodes and place the points of your cones directly on those areas, then the cones can provide isolation.  But it's very tricky to locate those nodes.  Use a stethoscope.  Or I suppose it is easier to choose another type of footer.

Get Symposium Ultra Stealth the best ASAP.

IME, SRA VR platform under my Woodsong Garrard 301 was a night and day difference. I am sure that there are other excellent devices but Silent Running Audio has 40 patents and three companies. One for Audio, one for Medical/Science and one for Military. If nuclear submarines and CAT machines use them...
The improvements in sound were as profound as any upgrade in my system. I was floored.