I have the 1a-!z and the user manual.
Plug the sub-woofer in the RCA plug socket under the center channel center- bottom row in the group of 6 RCA plug sockets just left of the 5 speaker wire terminals.
The manual is 56 pages and available in exchange for good Karma. Where do you want it sent?
Failing that, I'm wanting to identify the performance of the D/C converters on the 1a-1z. This isn't indicated in the user manual, have you been able to find original marketing information on this unit? It's not available on the nakamich web site.