First thing to know is that SACD (i.e. DSD audio bitstream) cannot be filtered. This means that you cannot apply DSP to the SACD/DSD audio such as EQ, phase, crossover, etc. Any 2-channel SACD disc is going to have to be played full range with only 2 speakers. If you do see a device that does true bass management with SACD, it is either converting the DSD to PCM so that it can apply a crossover filter, or it has a completely analog crossover (not likely). You can actually buy an analog crossover (such as the Bryston 10B) if you wanted bass management for all 2-channel music -- this would require you to disable the SUB in the Oppo speaker configuration. Otherwise, you’ll have to just live with Left/Right for SACD.
CD and other 2-channel hi-res (i.e. 24/96 or 24/192) can be filtered through a digital crossover. You might try going into your Oppo settings and configure your Left/Right speakers as "SMALL". This would force the Oppo to use subwoofer for any 2-channel music output. I would also disable any filter setting in your Pre2 preamp. You do not want a crossover filter applied on top of another crossover filter. In your situation, it could be that the Oppo is not doing bass management for 2-channel music.
I would highly recommend NOT using your 5.1 analog preamp to do any crossover/bass management, unless you are sending a digital signal to your preamp to be processed. In my opinion, the Oppo 103 is lacking in audio quality for your system. I would maybe suggest upgrading to an Oppo 105 (or the upcoming UDP-205) to get the superior audio boards. Or look at a higher-end home theater processor. A processor would do bass management for 2-channel audio (except for SACD/DSD).
There are two HT processors I know of that will accept SACD/DSD through HDMI. Both of these are architected to be somewhat future proof. They have already have had upgraded HDMI boards implemented to support 4K & HDCP2.2:
Emotiva XMC-1 (Dolby Atmos & DTS-X support coming soon)
Krell Foundation (superior audio, but much more expensive)
One other processor option you can look at is the Marantz AV8802A. It not really future proof, but it can play DSD from files or HDMI inputs. I would put it as much better sound quality than the Emotiva. It’s difficult to know if the Marantz is better than then Krell Foundation because the Marantz has fully discrete audio stages (HDAM). The Marantz has a whole lot more features than the Krell (internet radio, straming, Pandora, etc.)
For me, I am mostly a movie watcher, but I am very critical on 2-channel audio and love to listen to music. I went a different route and got a Krell S-1200U processor to do all my audio processing and generation. I decided that having a superior audio stage in my equipment was much better then trying to get something that supported SACD/DSD. The Krell S1200 has a much better audio stage than any of my above suggestions, but it is somewhat archaic in the HDMI interfacing and operation. I am happy to live with those quirks if I get superior audio as a result.