The Decware amp is somewhat unusual in that its outputs are described as "floating," i.e., not referenced to ground. That is done to make it possible to bridge it into a higher powered mono amp (although there are other ways in which that is accomplished in other designs).
A consequence of that is that by connecting the negative wire from the sub to the negative left channel output terminal of the amp there is not a good return path to the amp for the signal provided to the sub by the amp’s right channel positive output. So it’s somewhat surprising that the results are as good as they are. And that may have been a factor in the hum issue as well.
I’m not sure what the results would be if you were to connect the negative wire to the amp’s chassis, as REL recommends for situations involving amps whose outputs are balanced or bridged. It would depend on the amp’s internal grounding configuration, and if the outputs are truly floating the result might be a big hum.
An inquiry to Decware might be in order, to ask what they would recommend regarding connection of that wire.
In any event, adding a second sub would eliminate that issue, as well as presumably providing sonic benefits. Regarding whether to connect the subs in that situation to the amp outputs or to the speaker terminals, while it may not make much if any difference it would probably be worthwhile trying it both ways. My perception is that in the majority of such cases people connect to the amp outputs.
One final note: I can’t tell from your post if the interference from the ST-70 occurred only when it was powered up, or if it occurred even when it was turned off (in which case interference from its power cable may have been responsible). But keep in mind that a tube amp having output transformers should not be operated without a speaker or equivalent load resistors being connected to it, at least when it is processing a signal.
Good luck. Regards,
-- Al
