The original idea of comparing directly against an 18" reflex is, that some manufacturers inflate their figures, some don't, some use "tricks", etc. That way it's difficult to "see" on paper what exactly you've got. An 18" reflex is pretty standard, if not standard. Some might do a little more then others but as far as my experiences goes they are all pretty much in the 96 -99 dB/W/m region and 97-98 dB/W/m in the flat working range. This should give an easy reference for most if not all experience levels.
On the other hand, I do most commonly measure indoors, in a fairly large space (450 m'3 / 16,667 cu. ft.) I tried it outdoors but that was quite a hassle. So here I actually need the reflex as a reference, to excite the same room nodes (a blanc).
As I do live in the middle of a city, measure-ing outside would require a large car, a long drive, a generator, nice weather, a friendly smile to alarmed law inforcers (or a permitt). As I said before, working on it, but that smile, its so hard
Personally, for Cubo Sub I would use lowish to medium Qts 18" or 21" drivers (0.20 -0.35). However, if it works in Cubo 18 and most of the times Cubo 15, it will work in Cubo Sub.
When the final measurements appear they will be the usual: 18Sound 18LW1400, Void Acoustics V18-1000, Eminence Kappa 15LF and another yet to be named 18Sound 18" driver. Love to get my hands on a 21". Currently adjusting Cubo 21 plans to fit the PD 2150 (I do the talk, Xoc1 the walk). As soon as I get my hands on a 3015LF I will try that one too, as it sims fairly well (minding it's an lightweight 15").
After measurements I'll be able to make a list.
Best regards
Cubo