The SUB15 is going to be a good sub, no doubt. It is a sealed sub, so it won't be a deep bass monster, but it should have very good mid-bass output, superior to the Monolith or SVS subs, except for maybe the Monolith 15". It is very nice looking.
The Monolith subs do give up some looks in favor of performance. The 12" and 15" are both excellent subwoofers. They will have very serious deep bass output as well. The caveat is they are big and heavy. Just moving the Monolith 15" around is serious business- it is 130 lbs. The performance is excellent, just read
the review. Here is a
review for the 12". The Audioholics review for the SVS 3000 series is forthcoming, hopefully it will be published by the end of October.