alternatives to spikes on wood floors?

You could try Vibrapods, that's what I use between my floorstanders and a piece of laminated glass on carpet over hardwood, suspended floor. Sounds very good to me.

http://www.soundstage.com/synergize/synergize011999.htm
http://www.vibrapod.com/Reviews/tas.htm

Harmonic Resolution Systems Nimbus pucks. Info at www.avisolation. They've worked wonders in my system.

I really wouldn't fret too much over this. Your speakers are probably heavy enough that the difference would be negligible. In fact the spikes could possibly make the sound worse, by exciting the floor boards.

Apcd discs from Starsound are the perfect mate to speed away the resonant noise and protect your valuable floor. Recommend replacing the steel spikes with the brass Audiopoints. You will hear a major improvement with these devices. They will not dampen, kill dynamics or slow down your Thiels like the rubber devices made by others.