Most liberal shows do revert to gutteral humor. In the San Francisco there is an afternoon drive-time show where two goofy 20-somethings talk about variants of sex and bathroom behavior. They do touch social issuse but only to demonize Republicans. One time I listened they when they were "baptizing" a person with the Roman Catholic liturgy in a toilet -- then they flushed. (I got sick and outraged.)
In afternoon drive-time I listen to Savage who is much more entertaining than Rush.
I don't consider those kinds of shows real "talk radio" because politics is a secondary focus of theirs.
I'll bet their advertisers would agree. Shows like that generate huge listening audiences but rather unspectacular advertising revenue because they do so poorly among prized target audiences (professionals between the ages of 25 and 55).