The Oppo will be perfectly fine as a pre-amplifier. Contrary to what you were told it has variable outputs (both rca and xlr). See here for their directions:
https://www.oppodigital.com.au/blogs/reviews-resources/15420449-using-your-oppo-as-a-pre-amplifier
In fact, it will beat the pre amp section of your Primaluna hands down. However, I am not quite sure what your plan is: do you intend to merge HT and stereo into one system?
https://www.oppodigital.com.au/blogs/reviews-resources/15420449-using-your-oppo-as-a-pre-amplifier
In fact, it will beat the pre amp section of your Primaluna hands down. However, I am not quite sure what your plan is: do you intend to merge HT and stereo into one system?