NMV amps can still have preamp distortion. They have preamp circuits, and preamp tubes, drive them hard and they will distort. Marshall Leads/Superleads have no master volume, but they do have "volume" knobs, and they essentially act like gain knobs with the amp permanently on 10. Turn the volumes to 1, and it's a 125 decibel clean that will shred your eardrums. Turn those volumes to 6 or 7 and it's still the same painful loud, but now you're getting into tone you would expect from Marshall because everything is starting to crunch up. The power amp distortion of Marshall non-master amps is somewhat of a myth. Original Plexis in particular have extremely tight power sections. They resist distortion until they just can't handle it anymore. A lot of that legendary Marshall tone comes from the phase inverter. Install a post phase inverter master volume on a non-master amp, and it will still get gnarly as hell, just at lower volumes.