If you like the basic sound of Harbeth, the SHL5 is, IMO, a superior speaker with a more coherent sound, a bigger soundstage and deeper bass. Harbeths are not known as rock speakers, but if you are satisfied with the C7 in that regard, the SHL5s do more of it.
If you are truly looking for something that will play much louder, I would look into the Coincidents or, if the rest of your system is up to it, a used Wilson Sophia. With the right gear it can sound very powerful, but it is a tough speaker to drive and sounds like absolute crap with amplifiers that can't handle its tough load. Good luck.