Musicxyz, us long-term Zu Def owners have always loved the authentic tone dense sound of the crossoverless FRDs in the design, but I am bowled over that you rate this improved design as superior to such uber speakers as the Wilson Grand Slam, esp. coming from your personal ownership. Looks like I really won't be able to resist the move upwards.
Upon getting bass integration sorted with the SpatialComputer Black Hole which the Def4s will improve upon further, can you comment upon specific improvements in treble extension/transparency going from the 2s to 4s, since this is the one area that could be reasonably criticised in the 2s.