Base audio upgrade for less than $200

[guzerone]

The short version:
- Add $676 to VO
- Install 4-channel Amp
- Connect speaker outputs of head unit to the amp input
- Connect the door speakers to channels 1&2
- Connect the woofers to channel 3&4
- Set 1&2 to high-pass and 2&3 to low pass (150Hz)
- Install Rockford Fosgate P1T-S tweeters in the stock tweeter location (they fit perfectly)
- Set crossover to -4 dB
- Install custom crossover (2nd order 3.5KHz) inline with door speaker

Sounds soo much better, nice and balanced with very good clarity.

The long version..

Coding the Hifi option gets rid of the signal processing that increases the treble dramatically (necessary to make the mid range speaker in the door produce enough treble). Once that coding is active, the highs are gone.
I tried using BMW OE tweeters and a crossover at 6KHz but those tweeters aren't very good, they have a rather high resonance frequency so I went with the Kicker tweeters. These come with a 2nd order crossover at 3.5KHz and 3 different attenuation settings, the -4dB setting produced the most linear output, i.e. it matched the mid speaker in the door in terms of output.
Next step is to cut off the door speaker at 3.5KHz via a passive crossover, fancy term for an inductor plus a capacitor. Basically install a 0.25mh coil in-line with the speaker and a 8 microfarad capacitor parallel to the speaker (after the coil).
I mounted the tweeter crossover as well as the coil+cap on the inside of the door panel. The coils and capacitors can be sourced at parts-express.com for around $20 total.

The power output of the head unit leaves something to be desired and the woofers need a bit more power than the mids to have the same output. So I installed a 4-channel amp. I picked the JL-Audio because it has differential inputs, that allowed me to connect the speaker output of the head unit straight to the RCA inputs of the amp with out any line converters. I picked 150 Hz as cut-off of the crossovers in the amp because that seemed to be the stock crossover frequency. You could connect all 4 channels of the head unit to the amp and that way, you can adjust the low and mid level via the Fader but I only used the front outputs.
I installed the compartment left of the trunk in an upright position (fab'ed up some mounting brackets out of aluminum).
The easiest way to connect the speakers is directly at the subwoofers, the thinner two wires go to the door speakers, the beefier wires go to the woofers.

Then I used a RadioShack Decibel meter connected to a Behringer USB DAC and to adjust the levels (amp channels 1&2 and 2&3). I used a software called RTGA for that (it does sweeps as well as RTA and it's free).
The resulting frequency response curve is actually pretty good for a car (there are lots of standing waves, etc. and the frequency response in a car is never nice and linear).

The drop-off at 10KHz is actually caused by the head unit and/or AUX input, I measure the headunit itself (AUX-in to Speaker-out) and it has an interesting curve.

All in all, I'm very happy with the sound, the stock woofers are better than I expected once they get enough power.