I put 6.5 inch subs on the doors and they sound great! As far as 8 inch subs on the doors, no way, they won't fit in the stock location using the stock covers. I prefer a stealth look so that's why I went with the door subs. You can also slide a Bazooka or tube style sub behind the air controls and under the stereo (take a look). The side panels behind the stick, below the air controls pop off. I'd also recommend dropping a sub into the front engine compartment along with a new amp!
Good luck, post a picture of what you end up doing.
-Steve
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2001 Boxster S
Desnorkled
Pioneer Avic D3
JL Audio 6.5 inch subs (in the door)
Pyramid 4 channel amp
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