There are no pre-amp outputs on the stock head unit.
You can get a speaker level to line level converter, but the sound quality won't quite be the same and there is the possibility you may have some noise.
Another option would be to use an amplifier with speaker level inputs, but again, they can be noisy. I'd reccommend a good quality one, but I have been out of that gig too long. I think it a safe bet to say stay away from Sony though. Autosound is the one area where their products have sub-par specifications.
Your best bet would be to replace the head unit, but the adapters work. If you go that route and experience noise (alternator whine for example) there are some things you can try to eliminate them.
Make sure you properly ground the amp per it's instructions.