Ok, the thing about setting gains with a DMM and a 0db tone it the output will be less (usually) than if you set it by ear.
"Why?" you might ask... it's because when setting it by ear (for all those except trained veterans that know exactly how to do this), you don't give a SHIT about the safety of your woofer, most people want that shit loud.
Where as when setting with a DMM and 0 db tone there is no variance, the speaker is not hooked up, so you don't have a flippin clue how loud it is.
THAT IS A GOOD THING, that quietness that you don't seem to like just yet, that's the sound of a speaker being powered by an amp that is at it's limits, but not beyond. Not beyond being the key phrase. If you set your gains with a DMM and a 0db tone, you CANNOT drive the amp to clip when playing music, it won't happen. (Unless you turn the volume higher than when the gain was set, or start boosting frequencies.) This is overall MUCH safer for the longevity of your woofer.
So while you may not like that the speaker is a little quieter, it's better this way, if you want more power sell your amp and buy a different, more powerful one.