Thanks to all for your advices and suggestions, and yes, I've heard that a lot of people gave up in using preamps, especially in the case of DSd which has been specifically designed, when Mr McGowan didn't sell preamps and he was absolutely convinced that that was the right way to go, to work in this modality.
And yes, the way is to wait for the arrival of my BHK 250, to listen what will happen then and in case to have a BHK preamp borrowed by my dealer and investigate what will be the differences, 6000 $ in addition apart ;-)...
Mario