what should i do to my current setup?

Right now I have a Pinoeer BEH-2200 UB Head Unit going to a Sony XM-GTX6040 amplifier. I have some Sony XS-R5743 4-way speakers running through the front channel (via High Level Input), and a random 10" Kicker Subwoofer (that i bought off a guy for $50) running through the Rear channel via RCA (my HU only has Rear RCA outputs). With all that said, Should I get 2 more speakers + amp and bridge the amps? or get a better subwoofer(s)? or what? My budget is $300 The Amp I have is 600w... 110 w per channel at 4 ohms, 150 w at 2 ohms for 3 speaker setup. It also says 60 w at 4 x 4 ohms.

well depending on what your random kicker really is you might do better to get a dedicated bass amp for it instead of running it off of a 4 channel. then you can put your front speakers on the amp too. just because your radio only has a rear output doesn't mean you cant run the front on that output too. you would also benefit from a 3-way crossover ov maybe even an EQ since you only have one output. you can use them to split your single rca into 3 separate outputs and raise your rca voltage at the same time