Completing a 3-Way Component Set

Adding a driver dedicated to the mid and upper bass frequencies relives the component set mid driver from having to do double duty as mid-bass and mid-range.  That is the entire purpose of a 3-way system.  And in car audio where you can easily dedicate an amplifier to just the mid-bass driver, you can achieve stunning improvements in low-end punch and definition.  Adding a dedicated woofer that is capable of significant output in the 80hz to 320 hz octaves can make a dramatic improvement to your system.  ANY good quality woofer will do this; I recomend something between 6" and 8".  It does not matter what size the woofer in the component set might be as it will now handle the next frequency bands.  In my car for example I use 6.5" Seas mid-woofers in the doors and 7" Scan Speak woofers in kicks for stereo midbass.  The idea is to match the sound quality of the two woofer sets so you don't "hear" the transition from one to the other, and careful crossover settings.  You gain from the added ability of a dedicated woofer and amplifier, not from different speaker sizes.

I plan to replace my Scan Speak 18W's with Extremis speakers as soon as I can fabricate new kicks and afford the drivers.  The Extremis requires a pretty large mounting depth (gotta get all that xmax from somewhere!)  By the way I know of at least one person who is using a single Extremis ported at 30Hz as a subwoofer - they are that good.

As far as the question about what to do with the passive crossivers in the component set, you need to use them as designed.  All adding a mid-bass driver will do is relieve the woofer in the comp set from having to play those frequencies.  It does not affect the remainder of the audio spectrum and your passive crossover is required to protect the tweeter.

Your theory is sound, pardon the pun, but the problem I'm having is that I just don't see having his existing midbass also doing vocals as taking much away from the midbass performance.  It just doesn't take much juice to do the mids.  And then creating the potential for x-over problems I think makes it more hassle than it's worth.

If it were me, I'd either seal the doors with dynamat since they're likely a sieve right now or build that kickpod for a dedicated sealed enclosure.  Either would do wonders for midbass.  Otherwise I'd use an 7-8" midbass up front to move more air than the 6", likely in a pod as well for a huge, traumatic midbass increase.

question - is it possible that your existing 6" door mount is being cancelled out by the speaker's backwave coming thru door holes?  Have you played with sealing that door up well?  Do you have odd-fitting speaker spacers?