What should I use between spikes and wood floor?

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

I second the audiopoints...got some for my dad...they are soooo damn flat that they are actually pretty hard to pick up.

I went a more custom route myself but didn't save any $...and I'm sure they aren't as good.

I use pennies with a little felt on the side making contact with wood floor. It works fine.

Take a 4lb hand sledge and drive those spikes in.