I use an NHT PVC Pro, not made anymore but a very solid passive unit. Any passive attenuator is going to give you stereo image problems at lower levels, and assuming the construction/components are good, they should all sound exactly the same. It suits my needs as long as I keep my powered monitors at exactly half of their possible volume, that way I can go audibly quieter at higher settings on the attenuator, avoiding the stereo image issues entirely.
As for the Mackie Big Knob, I've heard many (former) users complain that it has the "Mackie Sound" to it. If you don't like that sound, don't get one.