There seems to be quite a few going end to end, or near enough, there's always good loads getting on at Middlesbrough and Stockton when I've been on. Getting from Teesside to Tyneside on the train isn't always quick and easy, depending on when & where you're going, so a slightly faster bus would help.
Not sure what other double deckers are more powerful (or if that's the issue) or would provide much more seating.

Go North East make the decisions and I'm only commenting on what I've experienced - the other day I travelled on the first bus out of Middlesbrough at 6am and was surprised how busy even that one got. Just seems there's a perfect template there for expansion, by bringing back the old X9.