setting up sub via low level input and setting frequency crossover

[Murforce]

I don't believe this is true as i have configured the the sub digital out on my 800 at a lowpass crossover of 80?

It's not possible to set crossovers for digital outputs on the Devialet. You can only use one or two analog output(s). One for mono and two for stereo.

You can easily set this up with Devialets Configurator (in advanced mode) for the two RCA/coax connectors displayed on the right side next to the AES/EBU (XLR) connector. In your case a low pass filter @40Hz. You'd have to determine the best 'order' of the filter (steepness) yourself. This is easiest when you have/can lend (or invite someone over with) a measurement microphone and software like REW.

If you don't want your main speakers to produce <40Hz as well like you say you also have to configure a high pass filter under 'speakers'. You can set it for both cases in which your pre-out is active and not. I'd configure the high pass active only when the pre-out is active so you can easily switch back to front loudspeakers only using the bass button on your remote.

edit: configuring it in the Devialet is the easy part. The hard part is finding the best location in your room for the sub, finding the best crossover frequency, best filter order, and finding the optimal phase settings on your sub. If you do this right your sub will blend in in such a way you "can't hear it", until you switch it off. This is all almost impossible to explain in written text. So if you doubt you can do this and want the best results it is best you invite someone over who has great experience in these matters.