Orange AD200B Power output problem?

[SkinnyMike]

Hi guys,

So I have an Orange AD200B paired up with an Orange 4x10 & 1x5.
When I first got this amp it would output crazy loud dB volumes then the preamp and master were set to maybe half. (12 o'clock)

Now, I'm struggling to get half the volume when the master is set on full and the pre amp set to about 3 o'clock (Any more and the preamp valves are driving to much giving me distortion)

Any ideas what the problem might be? Could it be a classic case of one/all the output valves being shot? How can I tell if it's a valve?

OR

Could this be a much more serious problem?

Best,

Mike.