2bigears - I don't think there's a bad choice here. Of course you know where my vote falls, but I haven't heard the other. Owners of those would probably endorse theirs to the exclusion of all others too.
Have any Def. owners returned theirs because they weren't happy with them? Well, I had the Druids and traded them for the Def's. I really liked the Druids but suddenly had cash to burn. Then I traded the Def's for Def. Pros. Was I unhappy with the standard model? No, but I was looking for the last 5% of low bass which the standard Def's did not do in my room. They were flat to 30 hz in my biggish room, and dropped quickly from there. I haven't heard any other Def. owners complaining at all about bass and I suspect my highly perforated room is unusually hungry in the low frequencies.
Most people would be very happy with this, but my musical diet includes hip hop and electronica which contain significant energy below 30 hz. Unwilling to trade the strengths of the Def's for a whole, new direction the Pro upgrade was natural.
Zu's approach with the Pro's is to let the main drivers run to their natural rolloff at around 40 hz. This figure is actually around 50 hz in my room. So, I'm using the TacT 2.2XP to cross over between 50 hz and 65 hz, depending on material.
Going this route is a little more intensive, as the sub drivers in the Pro's are actually pro models with significantly more output potential, though their inherent response is uneven. Unequalized, they run to 1 khz, so EQ and XO for the subs are mandatory.
Why the longwinded story? To share the primary weakness I personally had with the stock Definitions. Again, most listeners wouldn't have this problem. But if you're like me, going straight to the Pro's, even with the start-up baggage, would be worthwhile.
The upside is that the system I've got going (shameless plug) can do anything, given sufficient space. Girl-with-guitar is portrayed with tearjerking beauty, Snoop Dogg will shake the entire house, Miles Davis will be brought back to life, Drum Planet will blow your freakin' mind, and 2 watts via 45 SET amplification is PLENTY to do any of this at any sane volume. No kidding.
This rig is without any meaningful limitation. If you don't need the last 5% of bass extension, the standard Def's will do the same.