SVS Subwoofer Isolation feet. Will I lose bass?

Unless you've invented anti-gravity there's no such thing as isolation. What we have instead is vibration control. And you're right, shakey rubbery feet will damp vibration, sucking up energy and causing you to lose bass.  

Not very much however. Most all of these things work great because they work at midrange to treble frequencies where the ear is extremely sensitive even to micro-amplitude effects. All you have to do is squish one to see the most you can compress it is a very small amount. Its measured in hundredths of an inch. Then you look at the amplitude swing of a woofer, its measured in inches. You don't have to be a rocket surgeon to know they're not gonna have a whole lot of effect on a sub, one way or anther.

Audiophiles however are not even up to the level of rocket surgery. So expect a lot of different answers.

I thought you promised to spend some time in the library, mc. By their nature springy iso stands ARE anti-gravity devices. Hel-loo! The best performance for a sub would be place it on a set of springs. Awesome bass! Goes lower and more dynamics. You can forget everything else.

I added the SVS feet under my 3 subs on a tile over slab floor. I had a problem with things on the walls and shelves rattling when the lowest notes were played at moderate volume. The feet solved that problem. Whatever the reduction in bass was is offset by less rattles and more definition in the bass.