Someone posted the following in the coding thread of the 718 forum... I thought it was interesting, but may only work with PCM4.0 and above. If true, this will provide some additional tuning capability in the already limited PCM...
Sound Quality Coding:
X431:
SYSTEM SELECTION->PCM->Hidden/Brush coding->Hidden/Brush coding
->Set_Loundspeaker_Test_Frequency->
*->Set_Loundspeaker_Test_Frequency--bass
*->Set_Loundspeaker_Test_Frequency--mid-range
*->Set_Loundspeaker_Test_Frequency--treble
FACTORY (# = lower limit Hz for the built-in crossovers)
Bass: 100
Mid: 1,000
Treble: 4,000
RECOMMENDED (Brighter vocals and better clarity highs)
Bass: 100
Mid: 1,000
Treble: 3,000 (or 2,000)
(You can have PCM running and music playing while making adjustments to notice the changes)
The higher end of all of these settings is set to 19,000hz from the factory, not any way to change this, coding will not show any changes.
These settings do not correlate with normal Hz crossover settings as someone better at tuning an equalizer/crossover would think they would.
What I have found is that the bass setting at 100 is perfect, any less leads to distortion; mid can be adjusted as desired and will handle changes down to 500hz without distortion, and higher will take away some warmth/depth which can be added from treble; treble shows the most noticeable change in the tweeters and mid speakers (door/dash/rear), this is why lowering the response Hz of treble brightens up the sound stage.
My car has the BOSE audio system and these settings did help add some depth/brightness to the mid-range; whereas before it sounded muffled/muddy, now there is more warmth and brightness to the vocals. This is testing out with a variety of music genres.
I have also added Burmester dome tweeters and center dash speaker, so pushing the treble down to 2,000hz made it sound even better for my car and taste. The Burmester speakers are of better quality and I will eventually change out most of the speakers for these. I did not want to make significant changes by adding any aftermarket stuff to enhance the sound, so this worked well for me.
I hope this helps some of you out and should open up even more sound tuning capability, especially for folks with more understanding of sound than I have.