Spiking speakers not designed for spikes?

Set em on some Cones.

You will have to experiment to find which works best. None need to be fastened, the weight of the speaker holds everything just fine. Also experiment with placing different things like MDF about 3" to 4" square or round directly under the Cones.

According to the Stereophile review the top piece is removable solid wood. You could try replacing it with something with more mass and/or damping. Granite or marble, lead shot or sand, etc. Same for the base if it can be removed or modded. 

Each of these will improve something you can hear. Its easy to make something better. What is hard is to make everything better across the board. For that you experiment with different combinations. The beauty of it is you set Cones underneath and try, you set something on top and try, but you don't go cutting anything until you have already figured out by trial and error what combination works the best.

Millercarbon is a smart human.  Back in the 80s, we did exactly that with solid machined aluminum cones under floor standers.  The effect was immediate.  We used a 3 cone system for each side.

Another possible upgrade is simple crossover and wiring work.  Keep same values of caps and find some decent Solens or better.  Almost any decent speaker cable wired in is an improvement over stock wiring.

wooden bases that I’m nervous about removing or drilling into.

Use "T" -nuts of the same thread as your spikes, and for good measure coat the outside of the "T" nut with liquid nails before you hammer them in, and you’ll be fine I’m sure.

https://images-na.ssl-images-amazon.com/images/I/81x9eKE2eAL._SX425_.jpg

https://images.homedepot-static.com/productImages/5954888a-159c-46b2-a760-a79dcc9a1c21/svn/liquid-na...

But remember, spiking a speaker into suspended floor is a no-no as the floor becomes a sound board for the bass destroying any tightness to it. They should be de-coupled not coupled on this type of floor.
Only spike a speaker into a solid cement slab floor that can't act as a sound board. 

Cheers George

what doe's 'spiking' mean?

Though solid cement floors are less resonant than are suspended wood ones, they are still plagued by seismic vibrations. Spiking is so 20th Century ;-).