Three Alpine Type r's 10 inch running off 1500rms...What other subwoofer would you compare this too, maybe Fi BL or RE SX or JL w7, I went with alpine type r's mainly because I didn't have much money at the time, if i could do it all over again i'd do somthing else, dont get me wrong i love the alpines, but i'm about to get a third one. Amp is MTX thunder 91002 bridged at 1ohm is 1500rms. What do you think they would comapre too, i know this is kinda weird question but....
Three 10's has roughly the surface area of one 15. If the efficiency were the same, and the enclosure alignment is the same, and the power to the array is the same, three 10" woofers will have "about" the same output as one 15.
To go from two 10's to three 10's you will gain less than 1.5dB - a negligable gain, and certainly not "hearable". Save your money, get the woofers you really want. To add one more driver, you'll be throwing good money after bad, IMO.
Additionally, that amp is NOT stable bridged into 1 ohm, only into 2 ohms. You didn't say which woofers you had, but neither the dual 2 ohm OR the dual 4 ohm will get you a 2 ohm load to bridge to.
Check the woofer wiring guide... With three dual 2 ohm woofers, you can get 1.33 ohms, (too low - not safe) 3 ohms, (not max power - which I *know* you are looking for, for some God unknown reason) or 12 ohms. Even I wouldn't run 12 ohms, even though your damping would be the highest of these choices, and the distortion would be the lowest.
With three dual 4 ohm woofers, you can get 2.67 ohms, (again, not max power - see my previous comment), or 6 ohms (my personal choice - a good balance of power and damping). You could also get 24 ohms, but you would get even less power than the 12 ohm choice above.
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It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."