By al means, try it with the speaker outputs. The plate amp will have no effect on the Stereomour's sound, and this is the easiest connection to make.
The downside risks are small, but there are two:
1) If the main speaker's impedance fluctuations are large, the frequency response fed to the sub will reflect that to a modest extend due to the non-infinite damping factor of SET amps. I expect this to be insignificant since room resonances will be an order of magnitude larger, but it is a real effect. You may need to re-optimize the speaker position, but you'll have to do that for the subs anyhow.
2) The sub amp will get the hum voltage from the Stereomour's AC-heated filaments. This will normally be quite small, and if you cross well below the 120Hz hum frequency the plate amp will attenuate that hum even further. But in unusual circumstances this may be audible. If it's a bother, then some internal surgery will be needed to obtain a line-level outout to the sub that does not affect the internal sonics of the amp. I'd be inclined to tap off a tiny fraction of the 249K grid resistor on the power tube grid, in order to keep the source impedance small so that the cable capacitance doesn't upset the amp's tonality.
