There are still a lot of subs that will work "free air" you just have to know what to look for. No one labels subs as free air or infinite baffle anymore. look for a QTS or total Q value of .5 or higher in the specs of the sub. Anything .5 or higher should be fine.

Give these a look: http://mobile.jlaudio.com/products_subs.php?series_id=29

I think I saw a pair on ebay for around $220. I'd really like to see someone try this direction. Properly installed, two of these could sound like a single 12" sub.

The W3 is a great all around sub.

If you've got to be surrounded by sound, the best thing would be another pair of the MB Quarts for the rear. At least then it would be tonally matched. I don't think MB Quart made 6x9" components or coaxials, but I could be wrong.

Being that you're not keen to the idea of sound coming from one direction, you may want to take the rest of my advice with a grain of salt. My opinions will usually support the purist listener experience: flat response, focused sound stage, accurate tone, and very deep bass with quick transients. If you're looking for something new, you may need to do some experimenting, maybe some research to figure out what kind of listener category you fall into.

2nd set of Quarts ordered for rear - where do I get the adaptor plates from?

I think with the quarts, an amp and a sub I'll be happy.

I think Jack ordered identical 5 1/4" MB Quarts for the rear.

You can make your own adapters out of MDF, or order these:

http://www.amazon.com/Universal-Brackets-Speaker-adapter-plates/dp/B001AXHTZU/ref=pd_cp_e_1

Are there any devices for introducing a slight delay to the rear speakers?

There is, actually. You can take an old Roland Boss DD-2 or higher, and use its delay function to drive the rear amplifier channels. I've always wanted to try, but there are signal processors that give more control, and many newer head units offer the feature. I still like the Roland pedal idea because it's less expensive. Here's what the pedal looks like:

It needs an amp?

Don't know, it's very small in size. I doubt is has much power to offer. It might be able to power the rears, but if you want them to keep up with the fronts, you'll need an amp. I also think operating the delay box at low level would sound a lot cleaner. Let the amp do the work.

Alright, something to think about when I get an amp. At least it can be done. Maybe done better with a new head unit? Would really like to keep the factory unit but only being able to play cds is a bummer.

A new head would make it simpler to connect and use from the front seat, but harder to install tastefully in the dash. Unless you use a double DIN unit, I think aftermarket heads look pretty bad.

I learned about the Roland pedal reading a car audio magazine in the mid '90s. They were very impressed with the quality and low cost of using the pedal. I think it was $120 new back then.