I second madglasser. For killing mature bulls, the best call is no call. The area being hunted and amount of pressure on the elk are key factors of course, but we pretty much only use our cow calls for slowing/calming them down after we have stuck'em. Stalking, getting in front/cutting off a big bull is our main strategy with virtually no calling, especially on high pressue elk units.
Last year I hunted three states: warmed up in OR, bugled only 2 times all week, had a guy on me within minutes. In Montana, cow called once to stop a big bull for a shot that was walking out of bow range. In AZ my buddy cow called a few times to a big bull that we were close in on, the bull left us asap, he called again on another big bull and the bull went the other way. No more calling from that point on and my buddy is a former guide who knows how to call and kill bull elk. For young bulls or satellite bulls, calling can be pretty effective, but for old mature bulls, silence may be better. Now with all the above being said, recommend using a diaphragm call first and I historically, but not so much lately, I have fired some bulls up with the Sceery Hyper Hot, which is what I carry to call with after I have connected with a bull, which does not happen for me every year like my buddy in Montana who has killed 5 or 6 nice bulls in a row, no calling at all, getting in front cutting them off every time. Nice bulls, all 280-330 class.
Good luck!
Johnny