I recently bought the IE6 and IE7 at the same time, and had both for a 4-5 days before choosing which to return. Obviously I can't give any long term feedback, but did get enough time to give them both at least 40-50 hours burn in.
Like you I couldn't find anything about the IE6 pretty much anywhere, apart from Sennheiser's own blurb.
After that period, I found them both to have a similar balance: The IE6 being (as advertised) more prominant and powerful in the bass - and they really don't lose much in definition down low, if anything. Both are very detailed and insightful further up and a huge leap up from budget canal phones, and the IE7 had an edge in resolution and soundstaging that would perhaps have developed further if I'd had longer with them. But I wouldn't say that there was a huge amount in it. As I intended to use them on a motorbike (where bass can easily get washed out) I opted for the benefits of the 6 over the 7.
So impressed am I with the range that I decided to shell out on the IE8 as well, which was originally way above what I wanted to pay. I'd say they're a very worthwhile leap up from both, and in another league altogether when it comes to the bass. The depth, impact and definition in the lower registers totally eclipses the other two, and any other IEM I've heard. Further up they're similarly impressive, and extremely detailed (although not at all bright with a darkish Senn house sound). Again I've only got 30 hours on them so these are only initial impressions.
Another thing I would say is that the 6s are by far the most comfortable (and lightest), but the 7s and 8s offer better isolation. I'll be keeping the 6s for commuting.