Those papers aren't that old particularly for lith. Give them a go without. Too much KBr will also stop the lith development. I've heard that Sterling can tend to have pepper fog which is cured with a little sodium sulfite....or so I've heard. I've been searching for some Sterling Lith for awhile.....have fun.
Also, lith isn't high or low contrast by nature, but has a much larger potential contrast range than regular printing. That's one of the reasons why I love it......