I like the way your speakers are looking. It will be a great set up when your done. The stripes and grain in that oak should really come to life and show some vivid color contrast too. As suggested, the tung oil is nice finish. It will get into the wood and the protection will be in the surface fiber of the wood. The oil penetrates and then cures. The film finishes don't penetrate but instead form a protective film that covers the wood. Both can bring out the grain in the wood and make your speakers look alive. If you have spray equipment then lacquer is a great finish. As Bill suggested you need some special things to be safe while spraying. A mask is a must and you need to have exhaust fans that don't pollute with filters as well as explosion proof lights. If the weather is nice and your neighbors don't mind you can work outside. Thinners and thinning is temperature and climate dependant. If you don't want to spray but still want a film finish I suggest Varnish. Spar varnish is very nice and water proof. If your interested I can tell you how I apply lacquer or varnish.

Bill