Strangely enough I felt the 6400 had more bass impact than the 8400. This was even before the I attached the 8400's pads. I compared them side by side once for days and it's actually a close one for me. The difference was very minor. I do remember quite well always thinking that the 8400 didn't have enough bass for me, but the 6400 did. I should point out that the 8400 has more sub-bass.
Katun can correct me if I'm wrong, but I think he also said the 8400 had more bass impact than the 6400 until he tried the 8400 pads on the 6400. Again, very very minor differences.
IMO I don't think either of them have much bass impact at all. A tiny bit. Enough for me to be happy with their bass.
Comparing these side by side is kind of a pain. The only thing that REALLY stands out is how more relaxed the mids are on the 8400. At first I felt the 8400 was a little more clear sounding, but it took specific songs and it was very difficult to tell the difference.
Best thing to do is demo them in a store side by side if you can