What are the benifits of getting the more expensive over the lesser? I have read about people having them tuned to 22hz instead of 25hz, what are advantages and disadvantages to doing that?
Besides being able to tune the PC+, it also uses a better driver than the PCi. I was slightly disappointed with the quality of the bass when I had a 20-39CS. There was a ton of it, and very low, but it was not as clean as I wanted, and the port made too much noise.
I now have duel 16-46CS+ and it is everything I expected. A ton of pure, clean, bass, and loud!.
If you can afford it, I would definatly go with a + sub.
The benefit of getting the 25-31+ and plugging a port to tune it is to get close to the performance of the 20-39+, without the extra cost. But you will lose a little bit of maximum SPL.