To maximize what you have as the moment I suggest put your separates to channel 1&2 and bridge the channel 3&4 for your sub. Just make sure na ang amp mo ay bridgeable sa channel 3&4.
Why sub on channel 3&4, is because most bridgeable amp has LPF on its channel 3&4 bridged.
BTW I assume that you have only 1 pair of GTO628. But if you have 2 pairs better use the V12 to power both separates then get a dedicated amp for your sub.