whoa, hold on a second...
the resistance all depends on the driver - not the amp. To get a 4ohm load or a 2 ohm load on the amp you need a speaker with a 4ohm or 2ohm voice coil, or a DVC speaker that can be configured to a 4 or 2ohm load by either wiring the voice coils in series or parallel.
amps produce their power at a given impedance - for example your 4 x 50w rms amp. It probably does 4 x 50w rms at with a 4ohm load on each channel. Now I'm sure it will also be able to handle a 2ohm load per channel - then you would be looking at close to double the power per channel. This is because you have halved the resistance.
When you bridge 2 channels of the amp together you normally get more than the sum of power from the 2 channels - so for example your soundstream amp might do 150w rms x 2 when you bridge the two pairs of channels. This will definately be at a 4ohm load as 99% of 4ch amps cannot handle a 2ohm load on a bridge.
sorry for the hectic rambling above - if you knew this then I appologise.
When it comes to powering the standard subs off your aftermarket amp, you have no choice on the impedence - they are 4 ohm drivers. To get the most power to them you could bridge 2ch to each sub. I'm not sure if they would handle 150w rms - prolly not
PS: on your question on the other thread about the sven2 - yes it does have a remote trigger. Try a rockford dealer in capetown.