If your DI is getting a good signal then you've already ruled out the input jack being the issue. If I recall correctly there is one triode section (each valve has 2 triodes) after the effects loop on the Terror Bass (have the schematics here somewhere). Have you tried playing straight into the effects return and therefore bypassing the preamp? If so, try now swapping the 2 valves just to make sure the post effects loop triode hasn't gone bad.
EDIT: Found schematics. Yes, DI and Effects loop are between the 3rd and 4th Triodes in the circuit, as is the volume control (volume is post DI and effects loop). It's still quite possible that this isn't a power amp problem. Try playing straight into the effects return, then swapping the valves around and repeating.
EDIT2: Sorry - just noticed you've tried changing valves... Change of it being a valve greatly reduced then. Could still be a component failure somewhere around that 4th triode, but looking increasingly like the power amp. Contact orange. They MAY still have spares for it... Otherwise it's a valve preamp...