Its not so much the magnitude of the lows, which isnt as extreme as its reputation might suggest, but the voicing (again with the VH4 - it has if anything more, but its hard to make it sound muddy, where I found it hard to make the uber sound clear). It has no control over the lows, they just blubber about the place, and the bass is bass, low mids and some mids. The controls are a bit odd in that the presence is a mid sweep control, mids are mids and treble is treble. However, that means lacking a real presence you cant dial in the clarity that so many of its settings lack. The treble is really dull as well, to my ears.
The ubercab on the other hand is a force of nature. Love that thing; my powerball shines through it. Even better with k100s instead of t75s.
Oh, small disclaimer - my play time on the uber was with my Legra MDV602 when it had a C-Pig in the bridge, and tuned to drop B. So the guitar was hardly shy on low end, but on the same day with the same cab in the same place in the same room the VHTs and powerball sounded great - heavy as $% AND clear.