Mojo, if by transparency you mean detailed and not vieled I'd say its a draw. Both subs produced clean transparanet bass as you would expect given how flat the DSP keeps their FR curves. The main difference which is not subtle is in how effortlessly the EP600 pounds out high SPLs when called upon while the Little Subwoofer That Could runs out of steam climbing the steep hills so to speak.

There was a difference with drums which I heard as a faster attack and decay coming from the sealed sub, or a tighter punch. To my ears I thought the 400 was handling those transients better. My brother heard it as well but he really loves the fatter sound of the 600 and consistently chose the 600 with all music and film scenes. I suspect what he calls fatter sound is pleasant sounding 2nd order harmonic distortion.

Keep in mind that much of that slap/punch depends on how well you have the speakers dialled in with respect to blending FR at either side of the crossover point and if you have the sub and speakers in phase up to 200hz. Also room resonance can play havoc with kick-drum.

John