I want to add a subwoofer to the trunk of my car. I have a 300 watt jensen from my old car. Would I need to bypass the current amp to use the aftermarket amp? Will this affect the sound of my speakers?

 

If its a TSX....
you could run parallel amps. stock- drive the door speakers and tweets. jensen for the sub. Splice into the stocks inputs to get an input for the Jensen.
Look up Mercman's thread on balanced input stages. the signal inputted to the stock amp is differential and not single ended so if your Jensen does not have a balanced input, you'll likely need his balun to do the conversion. I'd ask him if paralleling the stock amp together with his balun+your jensen is ok. The impedance, Z, seen by the HU may be to small causing it to strain. Mercman.... ? BTW, what's your balun's Zin?