Can foam padding isolate better than spikes?

They are doing 2 different things; the foam is isolating the speakers from the floor, the spikes are coupling the speakers to the floor. The first act to keep vibrations from reaching the floor, the spikes to transfer the vibrations out of the speaker as quickly as possible. Either can work and which is best is case by case depending on room, speakers, taste, etc. As always use what you think sounds best.

Another vote for Auralex. I'm using them in a studio system under Dyn Audience 42's to isolate and tilt forward. Great product.

I've used tubular polyethylene foam pipe insulation with temporarily great results. I would cut about 2 inches off the end of the tube and turn it on its side and place it under each corner of the speaker (20 pound monitor). Unfortunately, the weight of the speaker crushed some of the air out of the thin-walled closed cell foam within a few days, ruining its effectiveness. I also tried using it under each corner of a CD player with somewhat better results. Before the foam was crushed flat and lifeless, it seemed to drastically reduce noise floor and improve low-level detail. It seems to provide a fantastic suspension... for a few days. picture
I'm curious about how well that swimming pool toy foam tubing stuff would work. It seems like it would stay springy for much longer.

Has anyone tried using small sandbags as feet with any results?

Home Depot is a treasure trove for all these "McGuyver" tweaks.

Check out Herbies Big Fat Dots and their other options.