Why would you ever want to connect a sub to it?
It would be hard. If you just connected it off the input you would have to add power to the signal. You could run it in parallel with your speaker, but you have to make sure they are balanced. Your sub wont sound much different unless you put a filter on it. If you just want better sound I would just replace your main speaker or get something other than a practice amp.
I have a peavey 5150 heavily modded, with a marshall 4x4 red limited edition cab.