Im a bit of a noob when it comes to car audio so sorry about all the questions. At the moment i have 2 clarion subs (600W) and 2 speakers connected to a 600w orion 4ch amp. This set up gets me 128db at the most i can't get any more. I was looking at a 400w monoblock option audio amp for just the subs.. Will it make much difference? Or should i invest in bigger subs and a big mono amp for them?

You won't ever get a lot of volume out of those subs, they are a lot of show and no real go. Best you can do is get an amp thats RMS output is equivalent or slightly higher than the max output in RMS of the subs. I know for a fact the Orion amps are utter rubbish, best to keep it for just powering your speakers.

Can anyone recommend me some new subs? If possible around the $150 mark per sub?

DD audio direct currently have very good pricing on their 500 series subs. They are are a single 4 ohm subwoofer, and the 512 (12") is 500WRMS, and at $175 they are very competitively priced. IMO though i would find a good monoblock to run the subs, and i am assuming you are looking at two, given your current setup. I would recommend a jaycar monoblock amplifier if your after bang for buck.

Link to DD 512: http://www.ddaudiodirect.com.au/cart/index...p;product_id=51

DD also have a flat rate registered and insured shipping, and is only $19.95 anywhere in Australia, and you only pay shipping on the first item, additional items ship free.

Thanks