I've been using these a while now with the accudio, and I believe it does an excellent job with neutrality, although I add a bass notch or two and sub bass. But even without the bass notches I wouldn't mind it at all, very nice.
However, I have been listening a lot today with the default music player and no eq at all, but using the headphone volume control it came with. I find that if I set the ipod touch volume to about 80-85% and use the volume adapter to control it, it actually increases the overall treble in the same way the accudio reduces the midbass revealing the treble. What's odd, is that ironically it keeps a lot of the sonic properties that you get with accudio.
Basically, because the volume adapter is adding impedance to the cable signal, it reacts according to this graph:
If you look at my eq settings which reflect the sound of accudio's neutral setting very closely, you'll see it is essentially the same as this graph. Keep in mind this graph is a high impedance range, the volume adapter is probably a lot less based on what I'm hearing. So, what you get is a slightly boosted treble region as the graph shows.
With accudio the neutral setting reduces the mid bass/bass which relatively speaking would be like boosting the other frequencies, as in the graph. The corrective cuts that accudio makes are almost exactly the boosted areas affected by higher impedance.
Anyway, bottom line, I have found that at the 80-85% ipod volume level the volume adapter allows the 4R to sound very similar to the accudio neutral setting, thus requiring no accudio or eq at all. It is not identical, but moves in the same direction. The accudio has a more distinct and separated sound, but at the cost of the mids a bit, although as you listen to it for a while you stop noticing it really. The adapter is a more gradual or smooth boost of the default signature. So, while it isn't a flat treble boost, but rather as you see in the graph, it is less "adjusted" sounding than accudio. I'm not sure which I prefer, but I can say I'm completely happy without EQ using the adapter
It's like having a bonus impedance adapter included. haha.