Dave,

The 518 and the 500 only have digital outs. Your sub only has analogue inputs. You'd need to add an analogue stage at some point to control the volume, which at present is handled by your DSPs. You could add a 561 or 568 so that you could feed your sub from the analogue out and your DSPs from the digital outs.

I dn't know your budget, but I wonder whether you might not be better served by upgrading your speakers to a better DSP. Your sub is so large I worry that it is going to be underutilized in a two-channel system with DSP5ks. I suppose it depends what you are listening to.

Going to a DSP6k (they seem to be very cheap in the US when you can find them) or a DSP7k (or 7200) might achieve the better bass you're seeking but in a more balanced way.

There was a discussion about speaker hierarchy last year, you'll find the HHers thoughts here: https://www.meridianunplugged.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=130871

Sooloos, Roon, G series (with LPS upgrade), 800 series (with LPS upgrade), HD621, AC200, Prime and PS, DSPs old and new.

Purdeys, Moke, Leica, 'Blad. Only the best.