I learned the hard way with Tri Tip also. It is one of our family's favorites, but don't trust your local supermarket with this. Can you believe that the first one I ever bought was from the supermarket and packaged with the fat-cap up? Might as well have been buying a piece of leather.
Only a prime from a solid and trusted meat market, or Costco, will do IMHO. The last 2 I got from Costco were terrific. But even there, I am picky. I don't buy the 2 pack because butchers like to mix a great one with a "so-so" one. I discovered that Wednesday is the day our local Costco puts out a bunch of fresh cuts; on that afternoon there are always 3 to 6 single tri tips to choose from.