Here are the typical hum levels of Bottlehead amps, as we have measured them:
Paramour/Stereomour: 2mV
Paramount 300B: 0.6mV
S.E.X. Amp 0.27mV
The Paramount should be 0 to 6dB quieter set up for 2.5v filament tubes (2A3 or "2A3-40" types), but we haven't made the measurement. 2mV is typical for AC-heated 2A3 amps. Hope that helps a bit.
The Miniwatt N3 is spec'd at 1mV noise, the S1 at 0.8mV - an insignificant difference. I suspect the difference you observed was not typical of these amps; hard to say if the S1 exceeded spec or the N3 was extra quiet. Was this comparison done in the identical system and at the same time?