I thought bad room placement would require more gain, in essence trying to fix the problem with the volume knob.
REW says a rear corner placement is best for me. mid wall is a no go for me, says REW..
Regardless of what REW shows, you still need to decide if it sounds best and one thing REW can't deal with is the question "What happens to the sound if I move 12" in any direction?". REW uses one mic, we have two ears and our brain does a lot of error management. Our hearing also tells us when problems exist, whether we can define it/them, or not.
Phase cancellations, as William mentioned, can't be removed with gain/level, although delaying the sound from one source can. If you use an AVR with Audyssey, set the distance increment to the smallest available and adjust it one increment at a time. Denon uses 1 foot and .1 foot- when I used an AVR, I messed with that frequently, basically to train myself to hear the differences and I would watch the response on the screen while using REW, etc. Subwoofers with a variable phase control rather than a 0°/180° switch can only be correct 50% of the time unless speakers are moved, the listener moves or the room is treated.
In my case, the room's problems caused me to build a new set of speakers and when that wasn't quite good enough, I needed to deal with the reflections with acoustic panels. Comments that they're unnecessary aside, they did a great job of removing the phase cancellations. I didn't even bother to use my subwoofer (it was a long evaluation period where I wasn't even bothered by what the sub wasn't providing) until about the middle of last year. Oddly enough, I had to do almost nothing to get it to blend with the main speakers but I did use REW/other RTA apps when placing the panels and making sure the sub/system response was smooth. Ultimately, the sound is most important, but if it can't be placed in front, it may require some trial & error testing to find 'the spot'.
