How to isolate a refrigerator?

it's true that you can spend megabucks on super-quiet (always European) fridges, but recent vintage, energy-efficient domestic ones can be quiet enough (+ cheap) to fix your problem.

I used to turn my fridge off for "critical" listening (it's in the same "great room"), but I kept forgetting to turn it back on.

@Miltcharlie: Thanks for the timer idea.

Actually my Hotpoint fridge is not too bad for noise. I think it was original equipment that came with the condo in '93 or so.

The floor is not the only part the vibrations may be entering. The walls next to and behind the frig may also be adding to the noise. If you dampen them also, it may add to the quieting.
Along with vibration pads under the frig feet should do the trick

my <1year old fridge is LOTS quieter than the 20 year old GE it replaced...and has cut my electric bill by about 50kwh per month while being about 10% more capacity.

IF your current fridge is more than about 10 years old, and IF the sound deadening ideas don't cut it, you may consider a new box.

On my brothers boat, he sometimes will put a small (2kw) generator on the upper deck. Adding a 2thick of those large foam floor tiles made it work and cut genset noise drastically. Tiles available at any 'home despot' type store.

FWIW... A gas refrigerator has no compressor and makes no noise. They work fine and are usually sold to folks who are "off the grid".