possible upgrade

i recently purchased an infinity kappa perfect 12.1, and i am liking the sq on it and the ok spl, but i hear my friends pair of memphis 12's that are being underpowerd by about 100 watts and im like wow spl out my eardrum.  i was wondering, i  have a RF 400 watt amp birth sheet of 428, if i upgraded from my 350 watt perfect 12 to a single 500 watt memphis would i still recieve insane spl if im underpowering the memphis by around 75 wats, or what would all of you genius suggest if i want spl with 400 watt amp and probably a 12 inch woofer, your input is appreciated

Depends on many different things. Like your car. Also Box design is also very important.

i got a 94 bonneville, its a boat, and im using a sealed box, but i was thinkin about changing that for a ported box, well whatever they sealed box should be an equal bass for both subs right? im just curious about the bass performance and i may go for the ported box someday. input on the sub power would be appreciated

The ported Box will give you a lot more SPL. Sealed boxes are great for SQ.
Your friend probably has a ported box.

I used to use a ported box with my 12" Infinitys, but it was TOO much bass, and switched it for a Sealed Box. Now i get good SPL with great SQ.

ok i guess nobody understands my question, what my question is, will a 500 watt rms subwoofer run up to par with a 400 watt rms amp, or would i be extremely dissapointed

It won't be a disappointment at all.  The RMS rating is a reference to the amount of constant power the sub can handle, not necessarily what it HAS to operate at.  If you are giving your sub good clean power, it will sound good.  If you like the way the Memphis subs sound being underpowered by 100 watts, then there is no reason to believe you shouldn't like them at 75 watts under rms. 

running them at 75 watts under rms will not show much diff at all. go for it and let us know how it works!

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dream it, build it, fiberglass it

Just for your info, i run a pair of Diamond Audio 10's that have a rms rating of 500 watt each and i am only feeding them 300 watts each and they do great so dont worry about underpowering them, they will do just fine.

thanks for the input thats exactly what i needed to kno