Sorry, perhaps I should have mentioned my assumption in the other post... =)

I am assuming that to get a stereo output that the EQ would have to be split equally. You have 2 completely separate signal paths inside the DSP (or 4 if you set it up that way) and the EQ has to be applied separately to each signal path (else you get channel mixing).

To put it another way, I have a Nord Modular, which contains DSP chips to create sounds. Each DSP has a certain amount of processing power available. The more processing power you use per voice, the less the number of concurrent voices.
So if you have a voice that takes 25% of a single DSP's processing power, you can only play a maximum of 4 of them on a single DSP.

Now if the in car DSP is programmed in a similar way and each EQ band takes (for example) 5% of the total processing power then you can go from 1 channel (or voice from the above example) with 20 eq bands, 2 channels of 10 eq bands or 4 channels of 5 eq bands...

However, I have no evidence that the In car DSP is setup like that... =)

Jazz
(List 112, Mk2 12 gig #40. Mk1 4 gig #30. Mk3 1.6 16v)