Half stack help?

Hey guys so I have a carvin head and cab that I just recently bought and it's has been fine until recently. The only thing I have noticed is that sometimes while either playing or when I first turn it on it somehow doesn't have any bass at all it kinda sounds like a radio effect. This happens even when the bass nob is turned up or down. I'm pretty experienced with eq so I know that's not the problem. I'm just wondering if maybe it's a problem with the crossover in the cab or maybe the tubes are going out either way I need someone who knows more about that to lead me on the right path. Any suggestions?

Thanks,
Moaq

I wouldn't say the valves are the problem, more likely a dodgy resistor or cap somewhere.

Expensive to fix?

That kind of depends on whether you are the one repairing it. Any parts you need won't be expensive, or particularly hard to obtain.
If on the other hand you opt to have a tech deal with it, you will obviously have labour charges to pay etc. If you are not comfortable working with high voltage equipment, don't take a chance, let someone that knows what they are doing repair it for you. Valve/ Tube equipment is well known for biting inexperienced hands...