I ended up using Infinity Reference 6.75" drivers. I only wired the woofers b/c I have 996 6x40 amp which too has the filter for the door speakers like the stock 4x40. I was amazed at the low end I now get - even top down on the highway (mostly due to the response and sensitivity of the speakers). Not sure of the max available depth but I think the Crutchfield guide will be accurate. Know with using a baffle (as the one I posted above) you'll actually gain a little more available depth b/c the speaker moves out about 1/2" further than where the stock tuned port mounts the speaker.
Here's a guide to upgrading the entire sound system by Andy M over on renntech which has instructions on how to safely remove the door panels (especially if your MY is 98 or newer which has side airbags).
Andy's Guide
Good luck!
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