The ZDT, by simple virtue of having a transformer, will sound very different than the ZD. It should, if I'm not mistaken, have a "tubier" sound, since the primary explanation I've heard for the good tube sound is an output transformer.
Also keep in mind that having speaker-level outs is useful for more than just speakers. Stax transformer boxes run off speaker outputs, and are (in my experience) an excellent way to listen to Stax headphones for a lot less than a dedicated amp.
And I much prefer the 4 pin balanced connection. Makes more sense to me.