I know what you mean. I thought the sound quality would degrade by removing the G68 processor and connecting the Oppo player directly to the amps. Not so. While I prefer movies with the G68 processor, I prefer music without it. Maybe because the player's analog outputs are digitizes by G68. Or maybe because of the power supply.
The HT bypass makes a lot of sense. It is the solution I am leaning towards. Another solution is to connect the Oppo directly to the G57 stereo amp for front speakers and go through the G68 processor/G55 5-ch amp for surrounds. I have all these components.
My concern is to add another expensive component only to be disappointed. After all, adding a preamp is an option not a necessity. Because the Oppo has volume control, a good power supply, and a good DAC. Of course the goal of adding a preamp is to improve the sound. But couldn't adding a preamp make it sound worse? I am pretty happy with the Oppo sound overall. The improvements I would like are better defined bass and smoother midrange. A tube preamp may smoothen the sound. But if it weakens the bass I would not be happy.