Digital BW, The Print

In my journey to fine B&W from Nikon V-scanned B&W negs I've been
playing with an image that never printed well in 1975 using a fine
Durst and Agfa Brovira...Tmax (never liked that stuff). Important
personal image. 

So far I've gotten best results scanning as POSITIVE, then inverting,
filtering dust & scratches (PS7), applying light USM, increasing
contrast (the pos scan is very flat). Print QTRgui/2200.

I'm getting more life (midtone separation) in shadow-sides of faces
than I have so far when scanning as negative...the overall image
appears sharper, with less USM, because certain tones aren't muddled. 

Scanning B&W negs as pos is often suggested... helps with this
image...any idea WHY this works, if in your experience it does?