What does the ideal room even look like. If you can't Precisely answer that question, then you have no hope of stumbling across the right answer...and certainly not by looking at anything that 'seems like a good idea'. The physics will necessarily prove to be too involved to guess at. Nothing at all will be right until you get the bass (100 Hz and below) right. No matter how much you spend on the 100-Hz-and-Up solution - if you spend it before you get the bass exactly right, whatever 100Hz-and-up solution you've already implemented is Guaranteed to be wrong and will have to be entirely redone once the correct 100-Hz-down solution is established. No matter how much money you dumped into the 100Hz-Up, if you've already done the 100Hz-and-up thing, it will All have to come out and you will have to start that part over from scratch.
Also, the bass is very hard and usually expensive to get right even in a good room and is too poor a value to attempt to solve in a bad room. If you have a "bad" room (almost every room is bad), you should walk away from the idea of treating it altogether, period. You will have no other real choice than to consider getting a good room. But, there is more to getting the bass right than room volume (although that's a start). As I say, you will need to know what the ideal room looks like. Drywall over 2x4's are not it...neither are 8 ft ceilings. If you have either of those things, you are already starting off in a supremely expensive hole. And about 98% of all the "bass traps" out there cannot fix the problem (never buy Anything advertised as a "bass trap").
If you find yourself in the 'I-think-it-should-be-easy-if-you-just-follow-logic-and-common-sense' camp, then go to acousticfields.com and watch Every video that Dennis Foley has made over the years and then tell me how easy it looks...that should pretty much slap you around some. As others have pointed out, you can't approach the right solution (the right one being the only one that truly works) by guessing or without access to expertise in the field. The physics just won't allow an easy, affordable answer...that really IS a pipe dream.
If you cannot afford the right solution or you would rather consider the wrong solution because you know it's cheaper, then financially you are much better off walking away from all but the very least expensive treatments at the first reflection points and calling it a day.
