i used to hate tri tip for the same exact reason you mentioned. at the time i only had a grill and would throw it on there for a few hours or however long it took. I would get a nice tasty crust with no flavor in the middle.
I have since evolved into a smoker and learned the ways. Tritip is now one of my favorite pieces of meat. I can only speak for apple wood since this is all i have ever smoked tritip with but its great. Super juicy in the middle, nice bark on the outside. i do a basic seasoning that is crushed black pepper, kosher salt, light dusting of garlic powder. i rub with extra virgin olive oil first, then season. Sorry, im out of order in my typing.
i smoked it at about 225° till the thermo reads 135° and slice thin to make sandwiches. Man, the flavor throughout is amazing. If you want more flavor just marinade it. In the past i have used Italian salad dressing with GREAT results.
give it another go.