What should I use between spikes and wood floor?

Polycrystal makes some inexpensive floor protectors that don't degrade the sound. You can probably get them from The Cable Company.
Linn makes some they call "Skeets," and are supposedly very good.

I put a slab of marble beneath the spiked stand with felt pads beneath the marble to keep it from scratching the floor.
works pretty good, you can pick up the marble from home depot for 3-4 bucks a tile.

Instead of spikes I use common hex bolts with same thread as supplied spikes. They don't mark the hardwood and provide solid contact. Just be sure to thread a nut on the bolt for tightening.
I may be cheap but spending alot of $ on some esoteric feet strikes me as absurd when the local hardware store sells nuts and bolts for pennies.

go to audiopoints.com. That is Star Sound Technologies's website. They have a brass coupling disc. Will do just want you want it to. Looks great, as well. peace, warren