^ Here's the puzzlement though. You said that when you lowered the output from the OPPO you ALSO raised the Volume in you pre-amp to compensate. I.e., you are still listening at the same absolute output level from your speakers.
But THAT means your pre-amp is sending the same voltage levels to your power amps as before! Right? If the speakers are producing the same absolute output sound level, then their power amp MUST be receiving the same voltage input level.
I.e., if you really were clipping the power amps originally, you should STILL be doing so, because the increased Volume setting in the pre-amp means it is feeding the same voltage levels to the power amps despite the lowered output from the OPPO.
Now, it may be that you have NOT raised the pre-amp volume to fully compensate. But if you've raised it AT ALL, that carefully calculated -26dB math still gets mixed up.
I'm wondering whether the real problem here is that you were actually clipping the INPUTS to the pre-amp (which is the more normal situation folks need to worry about). If that's true, then you could probably accomplish the same improvement with a much smaller attenuation of output from the OPPO -- and lower Volume in the pre-amp as well of course to maintain the same absolute level of output from the speakers.
If instead, you are now listening at a LOWER absolute speaker output level, and hearing better quality audio, then that might very well be due to clipping the power amps earlier when you were playing stuff louder. But if so, you should have been able to accomplish the SAME THING simply by lowering Volume in the pre-amp!
Another possibility here is that your pre-amp has a poorly implemented Volume control. By lowering output from the OPPO, you get to set the pre-amp to a higher Volume setting so that the negative impact of its Volume control implementation is not heard. (Volume is implemented as an attenuation of the max output level, so the higher the Volume is set the less it is actually doing to the output signal.)