I then had a friend apply pressure to rack and the rumble stopped.
The gross sort of vibration you have, so bad a person pressing with their hand is an improvement, is gross both in frequency and amplitude. All the so-called 'isolation' stuff is tuned to work within a very narrow range. That's why one person gets great results, another so-so, another says it might even be worse. Nobody knows ahead of time what amplitude and frequency is their main problem. No manufacturer publishes that info either. So all you can do is shot-gun it. Which as usual everyone else here is happy to do. Buy! Spend! He who dies with the most discarded toys wins!
Sorry. But the answer has been there so long I built mine more than 15 years ago. Its called a sand box. Shallow little box, filled with sand (mixed with a little oil to hold it together and eliminate dust) and with a little shelf on top. The size of the box, depth of sand, and type of shelf you play around with and by trial and error are able to tune to your particular situation.
Seriously. Don't take my word for it. Don't guess- test! If I'm right then you should notice improvement from something as simple as placing a 10 lb bag of sugar on the rack next to the turntable. Then if that works you could set the turntable on two 10 lb bags laid sideways. Point is you can figure out for yourself what is going on, and what will work to fix it, and all without spending any money or even going out of the house. You could do it tonight.
Go and see.