Newbie sound out issue - MacBook Pro, Scarlett 6i6, Cubase 9.5

I’m having a problem with sound output - I had it briefly working, but have lost it again. I’m sure it’s a simple setup issue that I don’t yet understand. Equipment is:
Cubase LE AI Elements 9.5
Scarlett 6i6
MacBook Pro Retina - Early 2015, OS Sierra

I transferred a live concert DAT recording to Cubase - DAT SPDIF Out -> 6i6 SPDIF In -> Cubase. No problem with this.

The MacBook sees the 6i6, both as an input and an output. If I play audio clips on the MacBook (YouTube, Pandora, etc.) they’re routed to the 6i6 - I can hear them either on phones-1 or running Line Outputs 1-2 to an amp. No problem with this either.

But when I play my concert transfer back, I have no audio output. I can see the output level indicators kicking, plenty of gain on both R and L. But I can’t get any output to the phones or to the monitor outs. When I first set this up I had the same problem - tried lots of things and finally got output. Then I had to tear down and move the equipment, and now I’m back to Square One. Any suggestions greatly appreciated - this ought to be dirt simple but I can’t get it to fly. --Peter