Speaker Spikes and related items....

Hi all!  My Paradigm speakers (monitor 5"s and Monitor 9"s) have interchangeable spikes to use when you are putting them on carpeted floors.  Now I am going to put them in our new addition with hardwood floors.  I see many different options and products that have their own systems with spikes and bases to put the speakers on to slightly raise the speakers up and isolate them from vibrations!  All well and good, but are the thread size for the screw in spikes a universal size or do the thread sizes differ by the brand of the speaker.  Thanks for the help.  FYI - the reason for my interest for these things is I have to position one of my speakers over an HVAC register and I do not want to totally impede the airflow from the register!!  Thanks, Kevin
They vary.  Most speaker spike threads are M12, M8 or 1/4” 20 but there are exceptions for sure.  You can print off a thread chart from the internet or just take a spike into a hardware store to confirm your size.
Dill....I put Herbies soft something or other things under my spiked speakers on a Travertine tile floor.  Absolutely ruined the sound.  Herbie took back the things, and my system returned. All of these things depend on everything....they have to be tried to hear the results. Sometimes better, sometimes worse, sometimes no difference.

It is only my opinion, but I would be concerned about a speaker over an HVAC vent.  Lying over a vent will produce very dry, hot air in winter, and in summer the cold air will definitely cause condensation on the speaker base as it hits the warmer air in the house.  I feel you will certainly cause deterioration of the box, if not stress the electrical innards as well (dying out, increased heat/cool and condensation in the unit also.  I hope their is a way to move the speaker a foot or two in any direction (or if a not yet complete addition, placing the vent itself a few feet away.)
dseltz is partially correct. Heating the speaker deferentially (expand and contract) may cause the enclosure to crack eventually. Water will not condense on the speaker unless your are refrigerating it for some reason. You can get registers that have deflectors on them that will exhaust the heat out from under the speaker but you would have to get the speaker up high enough to get over it and heat rises so the speaker is still likely to get warm. I had the same problem once and the solution was simple. I blocked off the register. The air flow will simply be sent to other registers in the house and as long as you do not block off to many of them you will be fine.
Ante up for the Isoacoustics Gaia footers.  Simply amazing.  Several reviews can be found on this forum.
Yeah, I'm curious about Isoacoustis - it's too bad that for the weight of my speakers I need GAIA I footers. And they cost $1200 for a speaker pair. 
@dmk_hifi, the Townshend Audio Seismic Pods are available in different weight capacity versions (4-8lbs, 8-16, 16-32, 32-64, 64-132 each), yet all retail for the same amount. More economically-priced mass-on-spring isolators are available from Machina Dynamica, and are listed right here on Audiogon.
You might look into Superspikes:  http://www.soundcare.no/

Excellent and relatively inexpensive solution for wooden floors.  They supply many different threads.