All of the grounds coming off the battery are grounded to structural members of the chassy and measure less than 0.000 ohms of resistance between them(my meter only has 3 decimal places--there has to be some resistance I just cant meter it).
A ground loop(in car audio) is only caused with multiple grounds with different potentials between different components in the signal chain--if your reference ground in your headunit/eq/processor is different from the reference ground at the amps, you can get noise, or in extreme cases actually damage the radio if your amp trys equalize the voltage through ground sheild of the RCA's(good amps wont do this). Because of the noise canceling properties of the differential outputs of the stock bose radio I'd be suprised if anyone has gotten any significant noise from a 4 ch amp in this car--unless their amp was a huge POS, or the install was shotty.
The battery is the WORST place to ground your amps, or any part of any sound system. Think about it this way, if you have 100 different ground paths running throughout your vehicle which point do you think will have more voltage fluctuations/occilations, the chassy(roughly equivalent to a 4/0 AWG cable--According to Richard Clark), or the negative battery terminal where every circuit converges... There may be points on the chassy that have issues(weak welds, alot of factory connections), but generally a proper ground to the chassy is no more likely to have noise than a ground at the battery(and in some cases alot less likely)...
I install car audio for a living. :cheers: If anyone ever needs any car audio help feel free to PM me..:cheers:[/b]