How would it be possible to evaluate just the preamp section?
Wouldn't playing your kit with a preamp in the chain and the PWD volume at 100%, and comparing it with PWD running direct to amp accomplish this? I think the usual complaint is with the PWD digital volume control.
Btw, when I did this comparison, my system with an ancient Adcom pre I dug out of the garage sounded better than the PWD running direct. I concluded that the often heard criticism of the PWD pre section was not without merit.
John