This makes sense to me after my experience with the D2000 I have. Now, mine is full Lawton modded and with just a portable amp (E-11) and adding a ZO it just didn't sound great on the bottom end, and even adding a Magni to the mix left me lacking. However, when I hooked it up to a vintage Kenwood KA-5700 integrated amp (out of the headphone jack) it really woke them up to the point that they sound as good as my Signature Pro's now. Serious low end and a very powerful output.
I am looking to trade the Pro's for a D7K so maybe I'll get to see how they respond to the vintage amp.
Thanks for the input.