any suggestions on subs that would work. Seeing there are a lot of sizes etc. A bit out of my knowledge base.
Perhaps a Denon X3700 which would elminate the need for the separate DAC. You will send a very clean signal to the NC500 amp(a bit cleaner than the amp AFAIK).
The Denon will offer excellent bass management for dual subs and excellent eq on both subs and speakers. Worth ditching the standalone DAC IMO due to simplicity of everything you need in one package, and good objective performance. Otherwise you end up with a complicated hodge podge solution of various components with limited functionality. Almost seems silly to get a 9 channel AVR for a 2.2 channel setup, but the availability of a good basic high fidelity setup(bass management, sub out, eq) is so limited, and even when you find it, it ends up being a lot more expensive and possibly lower objective performance, and nowhere near all the convenient bells and whistles.
For subs, I would probably look at a pair of Rythmik E15HP2 or PSA S1512. Rythmik is available in piano gloss, PSA is either satin black texture finish or wood veneer.
Another huge benefit of the Denon IMO will be DEQ which adjusts low bass and upper treble to match lack of sensitivity of human hearing in those areas at lower volumes. The result is much more pleasing sound at low volumes.