Digital BW, The Print

Having closely inspected lots of 2400 work by multiple
  B&W photographers on various fine art papers,
including various Hahnemeules, I've never seen
banding. 

Since you're getting this problem at Calypso, one of
the best several labs in the country as well as with
your own Epson, either your karma's especially
irritating OR something's wrong with your
post-processing. 

What does Calypso say about this? They're probably
right.

Make a very simple B&W file with no post-processing
other than the most basic Photoshopping to convert to
B&W (absolutely no third party software) and see if
that eliminates your banding. Start fresh.