I've got an '05 Nissan Pathfinder with Alpine speakers, but I haven't replaced the stock 6" bose subwoofers yet. I have Alpine 4 and 2 channel amps powering everythng (the 2 channel for the subs) and I want to put in a decent subwoofer. I was looking at the Boston 110/112 and 210/212 subs and the latter seem much better, but they are rated 75-300 watts rms. The 110/112 are 50-200 watts rms which is closer to the (bridged) 140 watts that my amp will put out. I've heard that you want to power the sub as much as it can handle so the question is:
Is it worth it to pay more for a better sub if it's only getting 140 watts when it asks for 300 rms? Is it just going to sound bad if it's that underpowered, or is it okay to not max out the sub? Also, what's the difference between using a 10" or 12" speaker if I'm putting the same power to it?
Well if you want the most out of your 140 watts than I'd say go with whichever sub has the higher efficiency.
"I've heard that you want to power the sub as much as it can handle"
Not necessary at all. Use whichever sub you like the sound of best. Fact is, some of the more critical and experienced audioholics will send no more than half RMS rating to their sub.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.