I would have to say a definite yes I just had two of the 15's installed in my 98 Tahoe and it shakes everything and flex my doors and windows with ease and thats with just one Massive 3000.1. I am extremely happy with these and all i had to use with stock alt which is 145 amps, and two vmax charge batteries and i don't drop past 12.9 volts. These subs are awsome a very good investment.//content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif
You're comparing a 15 to an 18 though. Plus its all install dependent. If both subs are in ideal enclosures, no it shouldnt be louder than a BTL 18. Given the fact you are giving both of them enough power.
edit: Dont worry man. If you feed the DMX some power, you'll be happy