After several attempts, I finally passed the quiz on the operation, and got permission from Mischa to fire Solar Flare up!
In its new home in my rack, and running with 45 Globe tubes driven by 12AU7 tubes in SET mode with Resistor Load (a.k.a. Orange mode):
Driving my Grado GH2 (SPL 1mW: 99.8 dB, Nominal Impedance: 32 ohms), testing for noise:
* 2-stage mode and high gain, closing my eyes and listening carefully, I can BARELY make a noise floor.
* Switching to low-gain, I am not sure if I can hear it anymore or not.
* 1-stage mode and high-gain - I cannot hear it anymore! Totally silent.
On a semi-related note: switching between 1-stage and 2-stage modes requires power-off and waiting for 30-seconds (for caps to discharge).
Being a tube amp, it is important to note that different tubes may have more or less noise. I will report back after testing with different tubes.
Also important to note that 2-stage / high-gain mode that had the "most" noise floor - was still VERY low. This is one of the quietest tube amps that I ever tested!
Usually, noise floor is a problem for me - but here it was definitely NOT a problem at all. That is just me nitpicking