In my eyes, it seems that B9s are off the road quite regularly. Whether it's a B7, Poppy Bus or an Olympian covering for a B9, there seems to be something out quite frequently!

Why other than a repair would these vehicles be off-road? Why would they be seen receiving attention in the depot regularly? An 'extensive wash' as I jokingly suggested above..?

Don't get me wrong, the B9s are great to ride on when they are out in service... But how much longer will that be for? No bus is designed to cover such an extensive mileage, surely... Wrightbus state that 'this high capacity vehicle is well equipped to meet the urban passenger transport challenges of any operation', but it seems unlikely in my eyes that when stating that, they did not mean such a high profile route like Go North East's prestigious X9/X10?

Coaches that were once used on the service seem more appropriate to me - but perhaps the thing that's off-putting is their lower seating capacity in comparison to the B9? The coaches in Go North East's current fleet aren't specially adapted to serve long distance either, to my knowledge, but coaches can be adapted in such a way to cover the needs of an extensive operation - why else would coach companies have coaches rather than double deck buses? :p
I don't know why the coaches were removed from the "Tyne Tees Xpress" operation - it's before my time as an avid enthusiast. Perhaps it was due to reliability issues, I really do not know... But are you saying if Go North East could afford coaches that were specially adapted to meet the requirements of the services (i.e. wheelchair access, the ability to cover an extensive mileage on a day-to-day basis for years to come, etc...), they'd choose a B9 instead?