I have put subs in 2 different 2005 model 330ss's both of them I mounted the sub in the panel under the helm seat firing forwards. Kind of into your calve if you are sitting down. In one I put a single in it, and mounted it on the passenger side of the dual helm seat. On the other I put in two subs, one under the driver and one under the passenger side. I believe they were 10" subs.

I originally was worried the vibrations might be uncomfortable but it was not noticeable in either set up.

There are already starboard panels in that part of the seat. so it was easy to remove the panel cut it out and mount the sub, then reinstall the panel into the seat