I just received my Comply T-400 tips today, and have been listening to them for a few hours now on my GR07's. There are some aspects that I'm loving, some that I'm liking, and some I'm not too fond of.
Fit - First, the tips themselves only fit firmly when seated over the wider base of the nozzle. This makes me wonder whether the fit will be strong enough for a lot of trips in and out of my ears or if they will pop off on their own and get lost. On my first day of using them they are still snug and I have no complaints at this time. After putting these on and listening to a few of my 'test' tracks there was one thing I noticed... errr... rather I *didn't* notice, and that was the complete and utter comfort they provided. In all honestly I hadn't noticed it until I did, but after a few minutes of having them in the only thing I could feel was the cable behind my ear. The earphones had completely disappeared and I was TRYING to feel them. This realization took over my thoughts for a few minutes. I gave them the good ol "let's shake my head really hard" test and they did not budge at all, not even a micron. I also tried various things which has made previous tips move around - eating, smoking, yawning - again, no movement whatsoever. I thought the foam tips supplied by VSonic were good, but compared to the Comply's they feel like lead weights stapled to your head. These things have won my vote in the fit department for their comfort, seal, and stability. If you need something to stay in place while on the treadmill, at work as a jackhammer operator, or if you walk face-first into a lot of objects I would suggest trying these out.
Sound - Now onto the part where I scratch my chin a bit. As mentioned by others who have used foam tips - notably the Comply's - these do increase the bass of the IEM quite noticeably. Ok, they don't really increase the bass, but between the better seal and the soft foam muffling some of the highs they lend a rather low overall tone. However, I never had the impression the GR07's needed more bass. I don't know whether I got blessed with a pair which is missing the epitomical sibilance, or whether I just enjoyed the clear and detailed highs more than I thought I did. To me this increased bass doesn't sound natural, it feels more like some kind of bass-boost and it can be quite noticeable depending on what you're listening to. When listening to some acoustic guitar and some lighter indie music they all sounded slightly faded and a bit muffled. The deeper parts of the guitar chords would also overpower the highs, so you wind up with the details being pushed into the background even more. This lower tone can work to your advantage though. I must admit I have come across quite a few tracks where the extra bass has been an improvement - some electronic music, dubstep (Thank you Captain Obvious!), various rock, and other genres may actually benefit from the increased low-end if that is the kind of thing you are looking for.
Overall I would give these a thumbs up and a positive recommendation when paired with the GR07. I cannot say anything bad about the fit or comfort of these tips and have actually been pleasantly surprised by them. I would highly suggest starting off with one of the multi-size sets if this is your first time purchasing the T-400 tips. The sizes are not wildly varied, but the small changes do add up to a noticeable difference in fit and comfort. Depending on the genres you listen to and your personal preferences the increased low-end may be a hinderance, a benefit, or a nonissue completely.