I too get less bass overhang and cleaner transients by crossing over at 120hz. As you say, imaging and treble sweetness improves too. This is probably due to the reduction in intermodulation distortion as much as anything, a very real benefit of biamping.
The initial impulse response of the big velodyne unit is noticeably more gradual than ideal, although it could be argued that the rise time of a deep bass note makes this irrelevent. My listening experience would suggest otherwise. Imagine a bass note, it's fundementals produced by the DD15 whilst it's harmonics are the product of the midrange driver in your main speakers.
The initial impulse responses of both units should be the same in order for the note to be portrayed accurately. The extra time taken by the Velodyne to get up to speed will give the impression of the subs being located behind the main speakers. This will give added warmth and body, at the expense of an increase in group delay, which will slightly soften transients.
In practice these effects are subtle, and indeed you may prefer a little delay. I just prefer other approaches.
