So I've done some tests with using different tips in either ear and estimating the difference in FR by mono-ing a song in an audio app, and panning left and right to see how each ear/tip sound, aswell as subjective tests with normal listening.
- The widest bore tips I have that work on the GR07 (wider bore tips generally are too wide for the GR07 nozzle to stay on) are a grey pair I got with some other IEMs. Not sure that model. They result in very treble-rich and uninhibited dynamics and clicks, though not what you want obviously since it also has the 6K peak uninhibited.
- The included VSonic tips (either of the ones in this picture, with the GR07 for scale, http://www.lendmeurears.com/images/00027227_01%20-%20Copy.jpg) with slightly smaller bore, slightly soften the treble relative to the above pair. Note that the GR07 Classic currently does not include any of the colored tips in that picture, only a few black tips with similar bore size and a foam pair.
- For fun I tried the smallest diameter tip I have, which is the tip for a pair of Klipsch S4i (http://images.klipsch.com/x2_1d8a94eb079cfff8088f63de47728e20_635042141559060000_medium.jpg). They fit, however snuggly, thanks to the soft silicon used. Seem to stay on great.
This match of tip and GR07 is actually really great-sounding. The GR07s smooth over and become a mid-centric IEM with awesome vibrating bass, however the treble is still pretty extended and doesn't roll off the very top as dramatically as you'd expect. It still manages to retain an overall pretty snappy sound, only now instead of hihats mainly crackling, they have a more mid-centric "cardboard"-y slap to them. The treble still smoothly gives them a mild click so it still sounds fairly defined and not overly veiled.
- Based on these findings, and how the GR07 responds going from the widest to very narrow tips, they seem to respond very well to tip size changes, becoming more and more mid/bass-centric as you size down, but even with very narrow bore they keep a balanced sound and the highest frequencies are there, though a bit in the distance. For most other IEMs I've ever used I'v hated the sound of narrower bore tips simply because it immediately veils the sound, but the GR07s are so crackly, defined and fast on the attack that they can punch through enough despite narrower tips, so they scale very well in this regard it seems.
So if you can manage to track down a bore size which is smaller than the included tips, probably a bit wider than the very narrow Klipsch ones, then you'd probably have a pretty good fit for the sound you're after. The tip length and material might create further variables but this seems to be generally true.