Let's get this straight. You want to use the OEM Sub wires to power an aftermarket Amp??

Though not impossible, this is not a great, or much encouraged way of installing an aftermarket amp. You're better off running power from the battery, as most amps have a pretty severe draw that the OEM harness will not be able to support properly. You can use the speaker output though. Most will tell you that a Hi-lo adapter is required for your amp. Not true. especially since you're only looking to add a little bass. SQ, concerns are minimal and your ear will most likely never hear the difference, especially in a WRX. The reason I say this is because almost all amps come with a hi-level (speaker cable) input. So the hi-lo is redundant. Let me know if there's more you need.