It's horizontal under the back deck

Hmmm... I have seen an installation done this way in a car that was very similar to mine (Taurus station wagon), the difference being that I have a space saver spare that mounts vertically back in one corner of the cargo compartment, and Taurus wagons with full size spares mount theirs under the rear floor like your Forrester.

My 10" subwoofer is mounted free-air in my (vertical) spare tire and the sound quality is (IMHO) exceptionally good. The one I saw/heard mounted flat under the rear floor did not have the sound quality I wanted. I don't know if it was the location of the speaker, the type of speaker, the amplifier, or just a general lack of tuning and tweaking (it took me weeks to tune mine the way I wanted it) but I liked my setup a lot better. This is not to say that yours won't come out perfect!

The other disadvantage to mounting in-floor in a wagon is that you then get to choose between carrying cargo or hearing your subwoofer. By the time the grocery clerk stacks three grocery bags on top of your subwoofer, your bass response is going to be gone! The acoustics in my car are so sensitive that if I even put the rear seatbacks in their upright position (i.e., not folded down flat) I have to retune everything and even then it won't sound right.

I have found, though, that by careful, thoughtful tuning and a willingness to experiment and break a few rules, it is usually possible to get any car stereo system that has decent components (enough power, good quality speakers and at least one subwoofer) to sound pretty good.

Please let us know how it works out.

tanstaafl.

"There Ain't No Such Thing As A Free Lunch"