I am late to this conversation, but here are my remarks, from input to output:
The potentiometer is usually best set at 100% if you are using an external level control. For the preamp with a choice of outputs, I suggest you try the 1.6v and 5v outputs and see which works best - the biggest difference will be where you set the control. Only in certain circumstances will it be necessary to change the balance between preamp gain and amp gain to obtain the best performance. I do not think you have this problem, I just mention it for completeness.
The 12AT7 is the main voltage gain stage. Your voltage measurements indicate that you have a weak tube (the plate voltage is higher than ideal). It is within an acceptable range for performance, but a fresh tube may make an audible difference. (If it does not, then at least you will have a spare on hand!)
The 2A3 is the power output stage, it provides some voltage gain but all of the output current. Your voltage measurments show that it is operating correctly, so there is no circuit reason that I can see for a short life. Most likely the tubes were worn when you got them. (In the unlikely event that this happens again, post back and we'll explore other possibilities.) The filaments of this tube are powered with AC in the Stereomour, and the hum balance pot allows you to cancel the 50- or 60-Hz hum; the exact setting will be different for each individual tube. There is a residual hum at twice the frequency, which usually amounts to about 1mV RMS at the 8-ohm output (1.4mV at 16 ohms, 0.7mV at 4 ohms). This will vary with different tubes. Remember, few meters will be very accurate at this low voltage.
The matched impedance of the output transformer (16 ohm setting with 16 ohm load) produces the most power. With 100dB speakers at moderate loudness, you probably don't need the maximum power even in your fairly large room, but keep that in mind as you listen - rooms and speakers vary greatly. The higher ourpur impedance also has the lowest damping factor, which will affect the frequency response of the speaker - not a lot, but it may be audible. This effect is strongest in the bass, which is also most affected by room placement, so the only way to judge is careful listening to each setting.
