Thanks for the comparisons....I think that settles it for me then. I've pretty much gotten used to the hd580 sound signature, so the 8400 is probably too bright and bass light for my taste.
I think the 8400 probably has treble similar to the K702. It may be bothersome for some. I think though that the treble was a bit more of a problem on the K702 I had compared to the 8400. The treble on the KNS-6400 feels a tad smoother and closer to that of the K601 I have. Maybe this is why the KNS-6400's signature felt more natural to me. What's weird is that when I compared the 8400 and the 6400 side by side, they felt pretty similar when it came to just their treble.
If you're happy with the bass on the K702, I'd say you'll be perfectly happy with it on the 8400. The problem is, is that there just isn't much bass impact and it's hard for some to get used to this. The bass on the KRKs is a bit different than what most people are used to hearing it seems. It seems like the KRK KNS-8400 might have better bass extension than the K702, but maybe not. I'll have to check the graphs.
Overall, the 8400 is close to being bass light, but not quite. Bass light to me is more like the AD700 and RE0. It's not even close to those. I guess for those that complain that the K702 doesn't have enough bass, it's best to stay very far away from the KRKs.
It seems the KNS-8400 may have some mid-bass hump, but I sure didn't hear that on my pair! The KNS-6400 definitely doesn't have one, which is a plus for me.
BTW it seems that the KRKs really do benefit a lot from amps and a good DAC. I was really surprised how much better my 6400 sounded when I got my HRT Music Streamer. Even seems to benefit more than my HD-598 for some reason.