Isolation Devices

Yes, felt wouldn’t defeat the function of any isolation device.

It might, depending upon its thickness, initially alter the base resonant frequency of the supported device, but should soon enough be crushed flat enough to make no difference for most devices.

Spring suspended turntables are a different matter entirely.

I really don't think they would but an A-B would test sounds good.

I use Herbies discs under factory spikes under my 160lbs+ speakers and they work well to allow you to move speakers and protect wood floor

Easy solution. Just listen. Get used to the sound with the felt. In a few days remove it to compare. Doing an A/B comparison right away doesn’t work as well.

Horrible solution. Where would this website be if people tried and listened instead of wildly guessing and pontificating? Don't listen to this wild man. Keep posting and asking until you find the one saying what you already decided to do anyway, and then do that, but under no circumstances actually compare by listening. Once you start down that road no telling where it leads.