The physical interface supports Jumbo Frames:
<60-9242> - MTU (bytes)
But it looks like the IP interface does not:
config-giga-eth 0/2.1)#ip mtu
<68-1500> - MTU (bytes)
I was advised by Adtran that all of their products including the 5660 with only do 1500 MTU on layer 3 / IP. Any Jumbo Frame support is restricted to switching / L2 only.