ya...I'd find a new expert.

the GTi is a great sub choice, but a lot of that reasoning is foolish.

as long as your gains are set up correctly, you aren't going to damage the woofers.

why not just have two of the subs on one amp each?
sounds like you are all over the place and buying into some audio myths and getting in over your head here.
you aren't going to want to switch a box with three subs in it on a whim to listen to a box with two 12's. that is a huge pain, and if you get a properly designed box, either setup should give you everything you are looking for.

 

there are a lot of good woofers that will do what you want it to, but for nice tight bass like your wanting, the amp is very important. you need something that has the right dampening factor to control that woofer properly. you'll get better bass response with a good amp and an ok sub than a good woofer on a shitty amp