After Martin breaks up with Maureen, he tells Frasier, "I may not have made it to Harvard, but I have been to the College of Love," to which Frasier replies, "Apparently on a Spillane scholarship!" This is a reference to writer Mickey Spillane, who wrote hard-boiled detective novels replete with sex.
Marty says that Sherry was on the stage and Frasier asks if it was Broadway. Marsha Mason who plays Sherry is a famous Broadway actress having appeared in many productions, particularly Neil Simon plays, to whom she was married. The line was no doubt added as a nod to Marsha.
This episode reveals that Maureen and Marty have been seeing each other ever since they started dating in the episode Police Story, which aired eight months earlier. Despite this, Maureen did not appear on the show in the interval, nor was it ever mentioned that Marty was in a relationship during all that time.
When Niles pluck Sherry's banjo, it sounds a B-flat. Given that the open strings on a banjo are G-D-G-B-D, he obviously plucked an out-of-tune B string that needs to be tuned up to B.