In my above post, I reference a thread which should have been titled:
"McCormack DNA-225 Platinum Edition Upgrades"
(without the word "amplifier" after it)
In that thread, the reviewer/poster states;
I would recommend a transformer-based passive or very neutral tube pre with this amp so that the full luster of the DNA-225 can be realized. In original configuration (100Kohms input impedance) the DNA-225 was a bit particular about volume control. While a high quality and finely stepped attenuator is still crucial, in my mind, it is not as much so given the new input impedance of 10Kohm.
Not quite sure what he is getting at regarding volume control, but his post implies the upgraded DNA225 may have an input impedance of only 10K ohms, which would restrict the preamps you might use to those having output impedance of less than about 500 to 1,000 ohms across their bandwidth.
Also, there used to be some posts around here from someone who had their DNA500 upgraded, although I could no longer find them. I know the DNA500 upgrades are not posted on the SMc website, but I believe there may be a few things Steve can do to improve that amp sonically, and also to provide a higher input impedance by using transformers. If that is a direction you want to go, you should check with Steve/SMc.