Hi vk9,
Good to know that you join the Auto family! It's very critical to know when to use the gears, 2 and 1 and OD/off. (For your A33 Power mode, I think you won't be needing it! hi hi hi)
In general, for day to day driving, D will be ok. Slam the gas to go. You car has to 4 forward gears, 1,2,3, and overdrive or final gear.
So, from stand still, you should be off in 1st gear, then you will feel a jurk and it's the 2nd, the 3rd then 4th. (Generally you can't feel it in your A33) Think of this, if you put your gear to 1, it will stuck at 1 and as your car runs, your RPM will goes up and up and up..
So, time for you to use 1:
When you are rolling down from a car park and you don't wish to apply breaks. This will enable engine breaking.
1 should put you at about 20~30km/h and 2nd gear about 30~50km/h.
I use 2nd when rolling down from Cameron.
For your ICE, I think just replace your head unit with a high power Pioneer or Sony.
50Watt x 4 should do the trick. I believe the Sub is an active sub, so just feed the source will do. Else, throw the ori sub and buy bazooka with a 300Watt amp on it.
My two cents...