I'm a newcomer to the "Dumble Way" and probably have about 25 hrs play time on my amp. (HRM 50) I decided tonight to play with the LNFB switch ON and try to get a real grip on this. Like Todd mentioned it's mostly subtle, but like everything else I'm finding out about this amp, all or most of the different switches and settings can have a big effect on another.
One thing I found is that the volume drop going from rock to jazz in clean mode is not there with LNFB switched on. And it opens up the low end really well. In fact a new favorite setting is the clean channel set to jazz with deep on and LNFB. With single coils mostly.
I'm really starting to see the genius of HAD's design. There are really very many voicings available. The way some of the tone switches interact with others is pretty awesome. I've had amps with lots of switches (Mesa Rectifier) but in the end it never really changed much from the overall sound. Not so with these amps.
The only problem I see, is not being able to get to some of the tweaking possibilities quickly. But then again there comes a time when I need to play the darn song and forget about obsessively messing with the amp.