Good evening,Charnwood on a wet,foggy day in Yorkshire !
I leave my Miscanthus till spring ,as I like seeing the foliage in winter,and possibly ,shelter for insects etc..then I cut them back to the ground in Spring,and thin them out if they are getting out of hand.Hope this helps..

I cut mine back when it has all turned brown because if I wait for spring, the new growth has already started and I can't cut it as low as I would like.

I used to start cutting them back in February. As we had some massive clumps I used the electric hedge trimmers. If any new growth was cut it did not seem to matter.