It all depends on several factors, which haven't been given.
"really loud" to you might not be "really loud" to me, or vice versa.
This setup won't be enough to win big SPL competitions, but in the same regard most people would regard "really loud" as being acheivable with much, much less.
It depends on your standards, which we don't know.
I could of course assume your standards aren't that high (by virtue of the fact that you are actually posting this question), in which case I'd say "you can get where you want with much, much less" //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
Likewise, the word "good".
What do you mean by that?
I would argue (by my standards) that this system would not be "good" for many reasons:
1) Your electrical system will not be able to keep up with the current draw from two absolutely enormous amplifiers... meaning you can make some severe and expensive(most likely around $1K total) upgrades to your electrical system (which have inherent compromises of their own, even including horsepower decreases), or you can leave your volume down to the point where the amplifiers aren't drawing the extreme current... in which case... why not just go smaller to begin with?
2) Even if your electrical system could keep up - let's assume for a moment it even has a chance - the resulting power output will create bass so loud that it will overpower your main full-range drivers. 2x, even 4x less power could have the same effect. It'll be so bass heavy as to not leave any regard for SQ.
3) You are talking about 3000 watts of power. If you can tolerate being inside the vehicle for any significant length of time with 3000 watts cranking on these subwoofers, that's actually a symptom of a serious inefficiency in your system... such as having the subwoofers in enclosures that are too small. In that case, throwing power at them isn't the solution - increasing the efficiency of the system is.
Remember Hoffman's Iron Law, which states the following three items are mutually exclusive in a subwoofer/enclosure alignment:
1) small enclosure size
2) high efficiency
3) low frequency extension.
You can't have them all, it's inherent, and unavoidable.
So plan wisely.
All this talk of power is rather ludicrous itself...
Do you all believe the woofer is so inherently stiff that it TAKES the RMS rating worth of power to drive the woofer to full excursion? The VAS doesn't indicate so, nor does it's BL. My experiences as an owner show me it doesn't... and I never would have conceived it would - it's a ludicrous amount of power.
In fact, what good would a woofer be, that "needed" that much power to be effective? What would you be gaining by purchasing such a woofer? //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif
It's simply a thermal rating. An upper limit.
Efficiency - which is what matters - is FULLY controlled by you, the enclosure designer. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
Remember, it takes 4x increases in power to effect only 2x increases in output... but the increase in current draw off your car's electrical system is very linear.
Plan wisely.
Power is not usually the key, efficiency is.
And particularly for new enthusiasts whose standards wouldn't approach justifying power of this magnitude, or even 2 of those 15XXX's.
I've had dB levels that cause my rear window glass, and moonroof to deflect a good 1/4", driving my rear view mirror to fall down facing the windshield.... with 400 watts as a subwoofer amp.
Efficiency is key.