I just recently bought a "Bugling Bull Yipper Yapper" and have had pretty good luck with it. I usually end a set with 5-10 mins of wounded pup and then howl a couple times before leaving. I've had that call about 2 weeks and called in 2 yotes using that sequence. Can't really say for sure it was the howl, could've been the pup. I don't think that call is made to be a howler but it works pretty good.
Also have a Knight and Hale howler. It sounds like a coyote with H1N1, never had any luck with it.