I cook a lot of tri tips- They're my girlfriend's favorite! However, she's not a fan of pink meat, and likes everything well done.
My method is this: Season with salt, pepper and garlic powder only. Keep it simple.
Smoke it low and slow until I get about 135-140. Wrap it straight- don't add liquids, because there's no need. Tri Tip has plenty of fat that will render out and make it's own juice (Refer to my picture). Take it to 175, then pull and rest it.
I'm going to try a reverse sear on my next one, but the above recipe is the one I've been using with great success.