That's a bad way to power a sub IMO. I hope the op wants yo bridge two channels for a single sub, not a voice coil. To power a single sub, a mono block PDX amp would have been much better..
Ok lets straighten out what has been said. Yes a mono would be more efficient and cheaper but, in NO WAY would I say it is BETTER. A 2 channel would make more sense though... Class a/b amps imo are best ran with subs when using very low power due to the increased sound quality when used. Yes, there is a difference and on other types of music unlike in hip hop it is very noticeable.