Rockbox

i agree

and FIR option would be great to, for the most powerfull players

I also agree. It wouldn't have to be too fancy, just switchable characteristics (shelving filters changeable to peaking filters).

But most important: I beg you for a LowCut (i.e. HighPass) Filter with the following specifications: 24dB/Octave (4th Order) and a tuneable cutoff frequency (between let's say 20 and 150 Hz). This would decrease (analogue) distortion at higher levels while most headphones are incapable of producing low frequencies - so why amplify them? Furthermore, with a well tuned cutoff frequency one would be able to boost low frequencies (even with a shelving filter!) without "mumble" the low end and/or stressing the Headphone(-amplifier) too much within the "useable" range.

This is a modification crucial to a sound-guy (/-fetishist) as myself and I would very much prefer it to an update in some game or plugin... ;-)

Changing filter types might be possible but I doubt you're going to get any more filter bands. The CPU is pretty limited on these things and, as far as I know, the EQ routines are as optimised as they can be. Maybe the Gigabeats might be able to handle more bands though.

The lowcut filter will be doable, but have you actually listened to see if this highpassing of high-level low frequency components is audible in the form of less distortion? Don't know if Ipods and Sansas will run very well with all five EQ bands and this lowcut filter at the same time too, since the lowcut filter will pretty much be comparable to another two EQ bands in CPU utilization.

Yes, it's audible. I tested the issue with preprocessed files (I'm a sound engineer). Besides, it will also slightly enhance battery lifetime (assuming same cpu load), especially with low impedance headphones.

And, in my opinion it wouldn't hurt too much to spare one of the EQ-Bands when the LowCut is activated (possibly by the means of a 3-way mode switch in the low band: LP24/LoShelf/Peak).

To original poster: what kind of extra filters do you have in mind? Ordinary lowpass, highpass, bandpass, etc? Is there really much use for these in an equalizer?

I mainly wanted to use 5 peaking EQ filters (actually 7 would be nice but i doubt the cpu can do that) a "3-way" mode switch as described earlier would suit my needs very well.

Something else to consider: there is an EQ package I have been using for winamp called "AIXcoustic - Electri-Q" which has an option to generate an IIR from input EQ parameters. This way, however many filters are used, once the coefficents are computed, there is minimal CPU used. I am not sure if that is possible on rockbox. If it is, you could have an IIR generated with given eq parameters (may take 3-6 sec to compute, then route audio signal through that; in the end less cpu could be used. again, I have no idea how the audio routing works in rockbox, if IIR would take too much cpu or anything else... just another idea.

Any kind of sophisticated filter design is best done on a computer for two reasons: 1. it's heavy computationally, and will take a long time on a target with a slow CPU, 2. it's usually a bitch to convert things like this to use fixed point calculations. Simple parametric filters like we currently use are easy and fast to calculate, in addition to pretty much being the standard way to do EQs. Also, Rockbox currently uses IIR filters for the EQ, it's FIR (or at least longer ones) filters we'll have trouble doing efficiently.
Also, it seems like the product you mention seems to use pretty much the exact same filters we do... (as seen here http://www.aixcoustic.com/index.php?id=28).