The sole reason why GNE don't use Coaches is because they have to cater to those with Pushchairs and Wheelchairs hence why GNE Bought B9's. I personally think there is nothing wrong with them and cope very well with the service compared to the old B7's.
I haven't heard of any problems with these buses at all since arrival in 2012, only time i've heard of any complaints is regarding lateness due to a crash or a minor brakedown on the A19. If you look at it since they started on the service there has barely been any brakedowns.
Plus you can't expect for a this sort of bus to be powering up/down the A19 on a Daily Basis, plus there not out all day as they are sometimes taken out of service to go back to depot to be refuelled unless there are some jerrycans kept in the bus station or an engineer brings them out from the depot etc. Happened today with the 50, CLS Station Manager went into the Travel Shop and got the Jerricans and then refuelled the bus as it was on empty. After about 2mins the bus was off continuing onto Durham.
I think it's great idea by GNE using that strategy to keep buses on the road all day for those types of services which have a high number in Miles to do on each journey.
Also if GNE were to use Coaches the only one to really meet the requirements is that coach Megabus use which is the Plaxton Elite i
http://www.flickr.com/photos/adammalarke...3159521784
Not clue on how much they cost each, but I would think it be between £250,000 and £600,000, I can't see it being anymore than that tbh and I can't see GNE ever going back to using coaches on the X9/X10, unless it was for National Express Work.