improving bass on large floorstanders....

Spikes for sure and it wouldn't hurt to try some different speaker locations. Good Luck.

Agree with all the comments above and add that some cables can help tighten up bass and your suggestion of Nordost is a good one, which worked well for me in the past. Another way is to have a better and more powerful amp.

Getting your speakers situated and leveled with spikes is essential. You can't hear whatever bass your speakers are capable of without them. Getting the speaker positions balanced between optimum soundstage and just-right room boundary bass reinforcement is essential. A high current amplifier certainly helps, as does turning the amp on for awhile to charge up the capacitors fully before playing music. For better bass also consider Transparent or MIT cable, though there are several that do bass well.

Like KR4 says, its the room. Start over with speaker placement. My favorite for proper bass response is the Sumiko master set method. Choose whichever works for you. I never found formulas of any kind to work for me. Good luck.

Tell us more about your system and room. What kind of floors are we talking here? That will be a consideration when considering spikes. If you are looking for tighter bass, you might want to look at your amp. Maybe an amp with a higher damping factor would help.