I used to have hum issues in my HT until I put an Jensen VRD-1FF isolation transformer in my cable line. It's a very common problem caused by poor grounding of the cable feed. If you run into it again you might want to try the iso xformer. The Jensen is pretty much the industry standard for dealing with this issue. If you run into it again try disconnecting the cable feed to see if the hum goes away. If it does that's the source.
Your comment about the smoothness of the Realta presentation fits with my memory of the situation very well. It's a very attractive film-like look. I really liked it. The current Oppo DVD upscaler has a more video-like look. It recovers a lot of detail but it just isn't as pretty as the 3930. There are some noise reduction settings in the Oppo that might improve things but I haven't tried them.
You are lucky not to have laser problems with the 3930. The 3930 forum on AVSForums is full of people who had this issue. Eventually somebody published instructions on how to DIY the repair for a few bucks, but by then I had moved on. You might keep it in mind if you get bitten by this flaw in the 3930. It's too bad really because as you say the build quality of this player is great.
I use a Roku to stream Amazon Prime stuff as well as Netflix through the Oppo. It has excellent ROKU stick integration and there is even a bundle price if you buy the stick with the Oppo. For $50 it's good deal.
Thanks for your impressions. It was interesting to hear from someone still using the 3930. It's the best DVD player I ever had. The second best is the Oppo BDP-83 which has a Mediatek chip instead of the QDEO.
My Denon DVD-5910CI uses the Realta and you explained the difference between it and the QDEO very well. I am in the camp that prefers the QDEO over the Realta, but both are very good depending on preference.