spikes for speakers or something else?

If you're lucky the spikes will drain resonant energy from those monster cabs. Get longer ones that'll pierce the carpet and pad thru to the sub floor.

How about larger spikes? With few exceptions, I find that spikes offer the best coupling. One of the most important issues in speaker placement is removing the ability of the cabinet from moving around in reaction to its own energy generated from the drivers. Take a look at this previous thread that might glean some information. My system has 176 lb.(each) speakers on a rather narrow footprints. Small spikes were good, large spikes are sounding better. Happy Listening!

http://forum.audiogon.com/cgi-bin/fr.pl?cspkr&1131824235&openflup&2&4#2

Coupling speakers is good, especially if you have solid floor, but isolation is even better in my experience. There are options for most budgets.