I tried the UE single flange tips too (had a few sitting around), with no foam inside, and they made the IE8s way too bright to me (if you can believe it). Fit-wise, they were great, but really messed up the Senn sound. So I guess I got lucky with those old biflanges. I just need to find some more, since I only have the one pair.
Was listening to the PFEs last night with the T4 and the Touch LOD, and man, they sounded soooo nice.