Comet partially replaced amber line by diverting through Little Eaton. They've had quite a reduction in service through there on the old Alfreton Red in recent years, from 3 buses per hour (hourly amber line, hourly 71, hourly nines (9.2?) to a now 2 hourly service on the notts and derby 71.
Surprised comet still exists, you'd think you'd incorporate it into the nines somehow so that it can be interworked. Think they added Little Eaton to comet simply as it had the running time, hour and half each way was generous, but hourly almost impossible.
I've not looked at timetables, but if I remember rightly nines departures from Derby are not evenly spaced, something that could be improved to make it more user friendly! I wonder if some use comet over nines as it stops in the bus station in Derby.
Of course, pre comet it was red arrow, where it potentially competed with pronto and rainbow 1 for pax from Chesterfield/Alfreton/Ripley to Nottingham. It did make sense to split it though for reliability issues if nothing else, and it's quite a simple change from comet onto red arrow at Derby anyhow.