Maybe at night (between ? and ? times) groups can't rehearse? No group practices between 1-5 am... Just throwing this out there! You would still get 100 energy each hour, but the rehearsal option would not be available. Whatta yah think?
Also, this would basically have the same kind of effect that the 600 energy cap would have, it would make it harder for newer groups to compete because they couldn't make up for staff difference as much by rehearsing more.