I'll also note that 'pure' harem stories eventually become repetitive and stale. Find new target. Control. Sex. Integrate with the others. Repeat. This limited scope is what brought Audiophilia to its well timed end.
The switch from harem to cheating seems like an attempt to give the harem members something interesting to do other than hang around waiting for their master to fuck them.
I'd agree with you that a formula can get stale, but I *liked* the fact that Audiophilia ended at a certain point. If you try to keep a story going indefinitely, it's going to have an unsatisfying ending.
