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.
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