The Mission MS450 is a good sub. Not the king of articulation or punch but it's got a soft and pleasant room filling bass response that doesn't sound muddy. It's a sweet deal if you can get it sub 50K.
In my room, it digs down to 22hz and has no problem pressuring a 450 sq.ft. area.
Do remember to let it break in if you go for it - at least 15-20 hours.
Sub level depends on how sensitive your speakers are - with my 92 db speakers, the sub level remains at approx 50-60%. With lesser sensitive speakers of 84-86db, i have to dial back the sub to approx 25%. This applies to all my subwoofers.
To sum it up, you have to set the subs gain and crossover to match your speakers bass extension and loudness. Any predetermined number may not give a pleasing result.
