@MattMoon: The STX is indeed a strange driver, not all the parameters seem to line up with each other. Now that either means I still have things to learn about driver parameters or it's a kinda fishy driver, or at least it's documentation. That doesn't mean that it isn't a good driver and I think it will work in either Cubo Sub or Cubo 18 Extended (I mean most 18" do) but I don't like it's fishy parameters.
As for phase plot, the only one I know of is a PD.154 in Cubo 15 Extra Extended, non Cubo 18's to my knowledge.
The high Le suggests a low crossover might be best, something like 80 Hz..
@Matthew123: The Wembley B Line has similarities to the Cubo design. Assuming the driver you mention was optimised for that design, would suggest it will also work in a Cubo 18 and than probably the Extended version. Warm and punchy is how I would describe most (but not all) Cubo 18's I've encountered or heard about.
BMS drivers are a fine quality, if a driver with good suiting T/S-parameters is available, it's a good choice.
@Majccco: If one would look at the parameters of Cubo Sub, Cubo 18 and the SB18 and the parameters of the mentioned drivers. I would say the 18ND9300 is best at home in either Cubo Sub or Cubo 18, whilst the 18G50 is best at home in Cubo 18 Extended or the SB18. So I think the 18ND9300 will sound better in a Cubo Sub or Cubo 18 than it will in the SB18, but it would also make me think the 18G50 sound will not improve that much when placed in a Cubo 18.
IIRC. Cubo 18 goes a bit deeper than the SB18 and will be flatter in response at the cost of some output, 1 -2 dB centered at 63 Hz for the SB18.
Best regards
Cubo