Quote Originally Posted by Matt Devo View Post
this is incorrect. The lower impedance of the Bose speakers only affects the amp driving them, which is still the same even if you replace the HU. The only thing you are changing is the (line level) output signal from the HU to the Bose amp. I agree that the Bose amp and speakers are not fantastic, but there's no impedance-related issues with replacing the stock HU with an aftermarket one.

replacing the Bose amp with an aftermarket 5-ch one (like the Alpine PDX-5 or PDX9) would provide significant benefits as well, even without replacing the speakers, but if the OP still has the stock Symphony II HU, that is the weakest link, not the Bose amp or speakers. A SII+ or RNS-E with an aftermarket amp would be an ideal start if aiming to keep an OEM look/feel.
Ahh, good catch. I was forgetting you simply feed a line output to the amp. Thanks for the correction Matt.