Base audio upgrade for less than $200
 

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).
https://u1vjeg-dm2305.files.1drv.com...&cropmode=none
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.
https://u1vkeg-dm2305.files.1drv.com...&cropmode=none
All in all, I'm very happy with the sound, the stock woofers are better than I expected once they get enough power.

Awesome write up just what I was looking for!
when searching the twitters part # P1T-S it returns a rockford fosgate part. is the part you meant Kicker 43CSC54? (the equivalent in kicker)

My bad, it's a Rockford Fosgate.

Great
and what about the amp the only one in that price range is this
https://www.amazon.com/JX250-Audio-M.../dp/B004FEIXRY

correct one?

That amp won't work, you need a 4-channel amp.
I got a used JL Audio E4300 on ebay for $75.

I see sounds like any 4 channel amp will work. Those are not available on ebay any more.

Does it matter if they are a class D amp?

found a JL Audio 400/4D not familiar with this topic at all.

That should do it, you might have to use the high-level inputs.

OK I got the amp but it looks like the one you bought puts out 40 watts x4 and this amp I got puts out 70 watts x4. the Rockford fosgate Tweeters are rated for 60 watts max rms, or 120 amps max wattage.

So do I run a significant risk of burning out the tweeters? 2 guys on amazon said they bruned them with a 400x4 amp so Im just wondering if I need bigger tweeters now.

No, the tweeters will be fine, they are rated higher (75w RMS I think).
Besides, tweeters usually only burn out if you overdrive the amp into clipping, basically turn it up until is sounds totally distorted.

Good to know

was there a set of instructions you used that was particularly helpful to code the vo $676?

there seems to be a ton of tutorials but very confusing if it's the same for e70

these are the instructions I found on e90

I see the $676 was added to the cas and frm but I can't tell any difference.

wondering if and which other modules need to be coded to default ?

someone said the pdc and cic need to be defualt coded but I do not see them listed. all i see is MPDC and MCIC both give errors when triying to code them.

and what gage coil did you get ? there is 18, 20, 15 ( never mind I got the 20 coil)