Got my monolith 10 in yesterday and can't begin to describe how happy I am with it, stepping up from a dayton 12 budget sub.
So just my observation on this post but a couple things. You do not need the y splitter for RCA signal on the sub. Just plug the single out into one of the rca jacks (even says in the manual). Leave the switches on thx setting for initial run though since extended will just make it louder (more distortion possible on top end). Leave the crossover at the highest mark possible since you have the amp doing the work (crossover should be disabled if thx is selected but won't hurt to leave it at max). Leave phase on zero unless you need it, you'll know if you need it. Put the port plug in if you feel your bass is going too low, that will help the roll off a bit higher. Sub in corner will also increase volume but do a sub crawl to figure out where you may like to place it (I was stuck in where I can given my room, but whatever).
Once done and placed, then run your auto calibration. If the sub is still too hot (your amp showing like -12 db correction) you can move the gain knob into the negative numbers to cut some off and then re-run calibration so your amp gain is -12 to -6 leaving you room to cut a couple db off in amp if you'd like. Every recommendation I've read is to make sure the sub is always at least -5/-6db on the amp. Then make sure all your speakers are set to small, raise the crossover (in amp) of all your speakers to 80hz if lower than that, and see what a movie sounds like at that point.
My impression was that the monolith was providing lower bass I just didn't have before, and the bass I did have was at a higher volume versus before. Just running a couple bass boosted test tracks made it absurd BUT by the time I was finishing my first movie with it, the sub had completely blended in and made the movie 100% better. Loud knocks or bangs were reinforced with volume matching the sound field so you could "feel" the door slam like you would if you were 6 feet from a real door slam. I would suggest just giving it a few days and a decent listen after you get it calibrated. still too much bass, turn the gain down.
Stepping up to a 12'' quality sub isn't going to solve your problem any more than trading this for a 10'' svs sub. At your price point these are all in the same relative performance group. Don't get too tied up in reviews and numbers and comparisons cause you'll start seeing BIG differences where there may be a 1-2db difference that could quite possibly not even show up in your room. I personally would also NEVER buy an audio product based on it's looks over it's sound production, but that's just me.