If you look at the pics on my web page you will see an 8' x 8' Cal Spa, I heard nothing but nightmares from people when my plan of having the spa inside of the solarium. The spa has been in place going on 6 years now, and I have no problems at all. For about 9 months out of the year, the cover is never used, durning the winter months, I put the cover on simply becasue it saves some on the heating cost, if not, the spa tries to aid in the heating of the house.
The rear speakers and one of the surrounds are only a few feet from the spa, and have never showed any issues from the spa. So in my situation, the spa is a mute point. YMMD.
