Glad you're enjoying it, Antony. Mine's just about the exact same alignment and I love it. Of course mine's just a big sonotube painted black, while yours looks like high quality furniture. But at least it matches the SVS on the other side of my living room
With a single subwoofer and all speakers set to small you should really just concentrate on the effects of the phase at the crossover frequency. Phase only matters when you have multiple sound sources playing the same frequency. In your case it will be when the sub and the main speakers are trying to play the same note. And this will be most noticeable at the crossover frequency.
Above the crossover point the sub is rolling off and any cancellation is reduced. Below the crossover point the mains are rolling off so again, any cancellation is reduced and the sub takes over bass duties.
If you've got an SPL meter then there is a fairly easy (if not quick) way to set the phase just right to get no cancellation at the crossover point. Here's how:
1)Use a free, downloadable waveform generator (like NCH tone) to generate a pure tone that is exactly the crossover frequency.
2)Save it as a .wav file and burn it to a CD.
3)Turn your subwoofer off.
4)Play the tone at a moderate level and record the SPL at your listening position.
5)Turn your sub on and disconnect your main speakers from the receiver.
6)Play the tone again. Adjust the subwoofer level until you get the exact same SPL level at your listening position as you got from your speakers
7)Hook your main speakers back up
8)Play the tone through both the mains and the sub. This time adjust the phase until you get maximum SPL output. You should be able to get exactly 6dB more than before when you're in phase since adding two equal sources gains you 6dB.
Note that the phase knob only gets you 180 degrees of variability. To try the other 180 degrees, reverse the polarity of the connection to the woofer. If you can't get to the speaker leads because the plate amp is mounted to the enclosure then I wouldn't worry about it too much.
But also consider this - turning the phase knob adds delay in the signal path. Delay is obviously a bad thing or we wouldn't worry about such things as "transient response" and "group delay". 0 degrees adds virtually zero delay whereas 180 adds the most delay. If you find that a setting near 180 degrees gives you the best phase response then consider switching the polarity of the speaker leads since this will give you 180 degrees of phase shift with no additional delay.
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