Well tolex isn't an absolute necessity. Mesa boogie and a lot of other boutique cabinet makers use nice looking woods which they stain and clear coat similar to how you would finish a guitar. If you're going to spray paint it with like a flat black, make sure you do any grain filling if necessary. And think about laying down a primer coat before colour, then clear coat it (and wet sand & buff) so it looks spiffy. That way, even with something simple like a plain black, it'll come out looking uniformly coloured and classy.
You mean use teak oil instead of a clear coat?
No, that wont work. Teak oil is a penetrating oil i.e. it penetrates the wood pores and conditions it so it's meant to be used on either bare wood or stained wood, not on top of paint/primer.
If you're using paint/primer you need to pick an appropriate varnish to go on top because the paint alone wont protect the wood from moisture absorption. Depending on the type of paint you pick, the most conventional varnishes you could match up with it are polyurethane or lacquer (acrylic or nitrocellulose).
Just a note... acquiring tolex might be more expensive, but it's also a lot less work and isn't very difficult to do. Finishing wood in a primer/paint/clear coat system is not only a pain in the ass to spray but it also has much bigger potential to screw up if you don't know how to do it well.
Another option you could look into is getting a auto body shop to prime/paint/clear coat it for you. Might cost a bit though.