First off, the quality of the meat is key. If the meat is of lesser quality, no matter what you try, you will not be able to compensate.
The marinade you used will actually start to cure the meat. You should have noticed that the surface of the meat changed in texture to a more hard texture, rather than the soft meaty texture.
You did not specify what seasoning you put on the meat, or how long after putting the seasoning on tgat you put the meat on to cook. Look at any of the rubs you have purchased, and the number one ingredient will be salt or sugar. The longer you let the meat set with salt or sugar on it, the more moisture that eill be drawn out if the meat.
I don't cook any thing at less than 250 degrees, and actually cook ribs at 275 degrees. Any poultry is cooked at 325 to 350. Brisket at 250.
I woukd suggest that you take a look, and search for recipies and procedures, at
http://thesauce.atbbq.com, where the bulk of everything is cooked on a Yoder Smokers pellet cooker.