I've browsed the Oppo's manual.
You must check to see if the sub is enabled. It may not automatically enable itself when you set a speaker to small.
So, set the sub to ON, set the front speakers to SMALL, choose an XO point, set all other speakers to OFF, check to see that DRC is OFF and the stereo outputs are set to FRONT LEFT/RIGHT.
With your tube amp off, make sure your sub is active.
Calibration comes later, when you have the Mini.
The 2x4 will allow the expansion of one more sub on its own output (you're currently using only one of the two mono outputs).
Edit: Actually, you could have a total of four subs, each on its own output. Output 1 will be paired with output 3 and output 2 with output 4. IOW, two per system. Plus, with splitters to those pairs that are co-located, you can have loads of subs.
You MUST have the Advanced 2.1 plugin, the 2-way will not do.
And as for using the Oppo's analog outputs connected through splitters to your Onkyo for movies... forget it.
You'll add undue complexity just to have the possibility of gaining some miniscule improvement during an activity that doesn't lend itself to such singular focus. "The movie sounded great, but I haven't a clue what it was about."
You will probably get better sound using HDMI anyway. Also, that miniscule improvement could be swamped by noise from all those analog interconnects. Oh, and one day, you may find that a channel is dead and you must twist the RCAs to revive it.
All that and the biggest problem is the 2-channel system has different downmix needs than does your movie system.
So, don't fight the current and go with the flow.
Edited by Oliver Deplace - 8/16/13 at 8:12am