Hello everyone,
Had the sound system upgrade done 2 days early (change in my work schedule...

). Satisfied with the results with a few caveats:
Without checking with me prior to re-installing the doors/quarter panels, they set the crossovers on both speaker pairs to '+3db' so the whole set-up sounds a little too bright for my taste. Setting the stock head unit treble control to around -4 cancels that out but as a result, the mids are slightly depressed so I increased them to +2 then, and then, and then... You get the picture.

They (BestBuy) said they'd pull the doors/quarter panels to (correctly...) set the crossover to 0db free of charge;
There is an engine-speed related whine, fairly faint but clearly audible at low volume settings. I think it's caused by the Stinger conversion boxe (which converts the line level output of the head unit to RCA compatible signal for the 4 channel amp) being set too high. If somebody here knows about this (D113...?

), please let me know. The guy who did the work at BestBuy said this was unavoidable but I'm not convinced.
About the Stinger being set too high: the max volume setting (on the stock head unit) that I can reasonably use (without risking permanent hearing loss...) is 16-17 whereas I could safely crank it up to the max (30) before the upgrade. The Stinger is feeding an Infinity Reference 475a 4x75w RMS amp. I'm thinking about reducing the gains on the Stinger so as to arrive somewhat short of the pain threshold at the max volume setting on the head unit. Again, any advice on this would be most welcomed (D113...?

).
I'm very happy with the Kenwood self-powered subwoofer and yes, it does fit transversely under the driver's seat, away from the feet of any eventual rear seat occupant. The sub very nicely rounds out the bottom end. I recomment it to anyone who's
not looking for a killer, gangsta-rap kinda of punch.
Overall, the upgraded speakers/amp sound clearly much, much better than stock. Audiophile quality sound is at hand once the (hopefully...) minor problems mentioned above are fixed.
Gringo