Both are late. For me in zone 5a, last 1/3 of May. In zone 8, I suspect you will soon be seeing growth, if you part their shorn culms and peek among them, in or by early May. (Alas, I no longer grow MuhliesÂwinters too cold; but for about 2 years I did, and they were always late, like the Panicums and some of the Miscanthus.)

My Muhly hasn't started greening up yet, either. I know it's still alive because I transplanted it not too long ago and it had a very healthy root system on it...took brute force to get it outta the ground so I could move it.
Reckon it's just a slow starter with regard to new spring growth.
I couldn't tell you anything about 'Heavy Metal'.

I learned last year not to buy dormant grasses. I bought a muhly and several Miscantus during the winter of 2004 that never came out in the spring or summer. I also bought a Heavy Metal, however, and it did finally leaf out in May. It has already started leafing out this year, with shoots about 2" tall now.

Have they emerged yet?