spl, 2 solo x 18 or 4 15 mtx thunder?

ok i had a few questions actually. how would you guys compare 2 kicker solo x 18" subs to 4 15" mtx thunder subs? SPL wise. i haven't heard the thunder subs in person yet.

second question: i have 1 dvc solo x 18, built my own box, competed, got it to hit a lil over 147 DB's, made the box about 2cu ft smaller, to 7-8ish cu ft, changed the amp hoping to give it more power, and i only hit 134 DB's. i changed the amp from an MA audio hk3000d(rated at 1200 watts @ 14.4 volts, 3000 @ 17 volts...i was deceived, silly me) to a hi fonics bxi2607d (2600 watts @ 1 ohm). both are 1ohm stable, and hooked to 1 ohm. only turned up high for competition burps. my question is why did it hit so much lower? is it possibly the amp? or making the box smaller that big of a deal? or is the port maybe too big? if it's smaller would it increase the pressure?

third...lol: what amp(s) would you put on a solo x? whether it be one or more, or daisy chained? i read that it's not good to put one amp per voice coil for daily driving. is that true even if the amps were tuned exactly the same putting out the same output? or if you think i'd have higher SPL going with the 4 15" mtx's? or another high SPL sub? thanks in advance for all of this

Five to ten feet? That's pretty vague of them.

it's a 2001 explorer sport. i'm trying to keep all 4 seats, but willing to wall it off in the back. the way i was thinking on the way home today, is to put my back seats down, then slide the box forward. it'll give me a little more room for a bigger box. cuz i want two of these, so they may need to be a little longer to make up for the volume. then i can fabricate some seats in the back trunk area behind the box. i rarely carry passengers, but in the times i do, they should suffice for the short traveling distance. i called the guy from ohio generator and he told me a 200 amp alt would be ok for competition burps. i hardly turn it up loud when i'm not at a comp. i dont have it installed yet as i havent quite found time. but it's sittin right next to me. 

yeah...5-10 feet does seem like a pretty big gap. but i'm almost positive that's what it said.

also, at 10cu ft, it said the port should be...i think it was 6 inches wide and 25 inches long? forget how deep exactly. but when i made the new box, i kept it 6 inches wide, but only came out to be 19 inches long.

thanks for the link. ima try that out