New loudspeakers & room digital correction service

[alandbush]

As you can see from the above I'm a bit of a novice AT THIS

Hmm, that suggests to me that you should take the easier route and use your current crossover.
As Thierry stated, you can then use either method to make the measurements. This will involve recording a measurement sweep for each of the left and right speakers with the crossover enabled and the sub active. There is plenty of advice as to how to do that, not least at the host of REW, I.e. the HomeTheaterShack.

It would have been nice to help further, but I could not find a suitable guide or forum thread that you could have used. I also don’t have a Devialet and I don’t have experience of measuring an integrated main + subwoofer. Phantoms are so much easier to measure in that respect.

It’s worth bearing in mind that EQ adjustments, at low frequencies, are, mostly, broad brush and as such you should not have to be too concerned about the minutiae of the measurements. I also recall that Thierry is amenable, within reason, to tweaking his supplied EQ to get the sound you like.

Best of luck.