[Otto]

Hi Eric,

If you use an RS SPL meter, you won't need a preamp; that device should be able to plug directly into the "line in" of your iMac. I'm not sure what the Extech is, but if it's a mic, it will most likely need a preamp.

As for which sub to buy, SVS, Velodyne and HSU are very popular and all should deliver the goods. SVS has a forum here, and they are well-respected and have great customer service. I would see what they have in your price range, and get one of those. I do not have a dedicated theater (and I don't have an SVS sub, either), but the thing that made the most difference in my bass was the BFD.