Please recommend a PreAmp for my 5.1 home theatre system

There is no problem using the Oppo's volume control. Once upon a time digital volume ocntrols were not a good idea as it cost you bit depth. These days, with internal 32 bit processing, this no longer applies. See here for a presentation by Sabre: http://images.computeraudiophile.com/graphics/2015/1216/digital-vs-analog-volume-control.pdf

I wouldn't do it

Why not? There is no technical reason not to use the volume control on the analogue outputs. Just look a that Sabre presentation. Times have changed.

I suggest an Anthem AVM-60. I think it is $3K new and can sometimes be found here used. Importantly, it has ARC built in, which is fairly easy to use, and can make a dramatic improvement in SQ especially in rooms that are not acoustically treated. I don't know how it will match with your amp, but I run mine with an Oppo 205 as a source. It provides a terrific home theater experience in a very large room.
Happy hunting
Al 

on paper it looks ok... but ask Oppo and they'll tell you for best results, its better to use a preamp for the volume control...... know that all volume controls does some harm to the signal.. its just that some does less harm than others.. analog seems to work the best thats why you see higher end preamps/processors incorporate an analog volume control into their all digital preamps.. My Classe SSP800 has an analog volume control for example..  there's many different designs in volume control, each one trying to better the other.. PS Audio claims they make the best one yet