Andy is a top bloke, and im sure he will be happy to talk things over with you. Ive also had both the single sub and now the dual sub kits, so if you want any impartial advice, then feel free to message me
To be honest it depends how much you like your bass and how much you like to feel it. The original single sub I had was one of Andy's JBL subs and lite box amp packages. The sound was very good but I felt wasn't quite punchy enough at higher volumes. I do like to really feel the punch of my bass. The new dual sundown subs are much more punchier. Obviously with the dual sub box though it's a bit more work installing it with the power cable going to the battery, so removal of sills and fittings are required.
If your just happy to have bass then I would say the single sub is an excellent upgrade. If your serious about bass then the dual subs will meet your needs.