Would tung oil help my bass cab?

Tung oil might stop the top coat from keying into the wood properly. A wood primer then undercoat would probably be best. Tung oil is a oil loaded resin as far as I can remember which gives a lovely finish to untreated woods and brings out the grain but not an undercoat.

 

Why put a coating on the inside?

 

I would check with what the product you intend to use recommends before applying tung oil, the products might not like each other. I used Duratex and I don't recall even using a primer, I wouldn't have applied tung oil first when building my cabinet.

I wouldn't recommend coating the inside. You will add a bunch of weight for no reason.

 

Is that Line-X coating? Compare that cost with Duratex and go with whatever is less. Coat the outside only, and no tung oil.

 

These bed liners can add a lot of weight to a cabinet.

Look at the weight per square foot for the recommented thickness of your coating. Based on the surface area of the inside, you can estimate what the additional weight added to the cab will be.

If it is too much, you might want to consider another product for the inside.

But the inside shouldn't need to be treated. If it's a ported cab, line it with acoustic damping material. If it is a sealed cab stuff it with damping material. Either way, the purpose of the damping is to absorb reflections and make the cab sound less mid muddy.

 

The inside doesn't need protective coating and the liner adds a lot of weight. I would just use acoustic foam or poly batting and call it a day.