I would try using the XLR balanced outputs for the Nuprime amp and using the RCA outputs for the sub. It looks like the Oppo HA-1 has separate opamp output buffers for both the RCA and XLR outputs, so this should not present a problem at all.
https://www.oppodigital.com/headphone-amplifier-ha-1/images/HA-1-inside-hr.jpg
If this doesn't work, your best bet is to use a splitter cable for the RCA output, something like this:
http://www.audioadvisor.com/prodinfo.asp?number=AQFLX-XMF&gclid=CNHCp_qJ1s8CFVKEfgodtokK9Q