We usually use Montreal Steal Seasoning. Simple garlic, salt, and pepper will work just fine as that cut of meat doesn't need much. Let us know what you decide to do.
If I am trying to emulate Santa Maria style:
Equal parts kosher salt and medium grind black pepper
1/4 to 1/2 parts granulated garlic and onion
1/8 part dried citrus peel
When doing something a little more...
Simply Marvelous Sweet and Spicy is excellent
Never had any bitter tastes. We sear a couple of minutes and then go indirect the rest of the time. We don't burn (at least intentionally) any thing in the rub. Try a few different ways and let us all know what works for you.
My version of the Santa Maria Rub: 3 tsp kosher salt, 2 tsp each of granulated garlic, crushed dried parsley and coarse black pepper. Reverse seared over red oak. Pretty much the only way I do tri-tip now days.
I cook tri-tip about once a week and I cook it only over oak and the best reaction I get is: Lawry's, Garlic Powder, LOTS of Black Pepper, Cumin and Paprika. I sometimes will throw on some brown sugar, it depends.
I just did one of them on saturday. I used coarse ground black pepper, kosher salt, granulated garlic, and cajun seasoning. Cooked over lump with a couple of chunks of hickory.