That’s really helpful. I’m going to try and see what I can do re-cheap isolation/platform stiffening before I can move everything around! Given your speaker is about twice the size of mine...it shouldn’t be the speakers!
at the risk of repeating myself, try putting your TT onto soft squash balls. Either cut them in half or sit whole ones inside of something like a curtain ring.
Here is my TT which is pretty close to the left speaker. I've not noticed any issue with vibration or feedback.