Your 3.7's are a very fast and detailed speaker. You need to make sure you get the right sub or you will not be happy. Since your speakers are time and phase correct, I would keep it that way. The Vandersteen 2qw sub uses 1st order xovers. Its also very adjustable, and is meant to be used with the type of speaker you have.
" Columbia interconnects and type 4 wires "
The Columbia's exceptional, but the Type 4's are not. If you replace them with Type 6 or 8, you'll be glad you did.
" Columbia interconnects and type 4 wires "
The Columbia's exceptional, but the Type 4's are not. If you replace them with Type 6 or 8, you'll be glad you did.