Don't have any experience with the Nova but I'm using the Decco as a preamp and DAC with a tube amp and it's a very sweet combination.
There are a couple of options for adding a sub. You can use a Y-adapter like this to go to both the power amp and the low-level inputs on the sub. If your sub has speaker-level inputs you can connect cables from your amp speaker terminals to both your main speakers and the sub.
There are a couple of options for adding a sub. You can use a Y-adapter like this to go to both the power amp and the low-level inputs on the sub. If your sub has speaker-level inputs you can connect cables from your amp speaker terminals to both your main speakers and the sub.