Marantz AV7705 AV Receiver: HDMI out to external DAC

OK, wait, wait. I get it.

A digital HT bypass could work ONLY IF the sending unit could control the volume of the digital signal before it hit the DAC in the receiving component.  But it's not possible to control the volume level of an encoded digital signal -- at least not at the level of technology we’re talking about.

So that’s the reason why HT bypass will work only with decoded analog signals. Now that I get it, it’s head-slappingly obvious.  Sigh.

So, like most other people in the world of multi-channel, I’ll have to live with an AVR’s preamp & DAC for all 5.1 channels.

@erik_squires Yeah, you’re right that a 5.1->2.0 mixdown wouldn’t do the job. But as mentioned above, even the Theta board wouldn’t get me past the real sticking point: A digital input can’t be configured for HT Bypass. My mistake was not thinking hard enough about the issue to realize the existence of that constraint.

So one option is to simply route the Oppo’s FL/FR analog output to any T+A input configured for HT bypass. But from a system-design perspective, it makes more sense to send 5.1 audio (via HDMI) from the Oppo to the Marantz. That way, all FL/FR analog signals arrive at the T+A’s HT Bypass input decoded and pre-amplified the same way. Again, only the T+A is in the signal path when playing stereo content (aside from things like phono stages & disc transports).

Regardless, I appreciate your input. I’ll probably research the Theta board further tonight. Even if I never need one, it’s an intriguing product.

OK, solved the problem. Save for future reference.

Stereo sources go directly to the T+A amp, which drives the main speakers and the wireless input of the sub (which is processed by the sub’s internal crossover). No home theater components in the signal path.

But the Marantz is the gatekeeper for all multichannel sources. It sees the front speakers as "small," and assigns them a fairly high crossover that is complementary to the LFE crossover. The LFE channel goes to the sub’s LFE input (which bypasses the sub’s internal crossover). In this mode, the T+A amp is still nominally sending output to the sub, but it receives so little low-end content that there’s virtually no redundancy between the sub’s two inputs. This configuration also drives the sub, during MC playback, with a true LFE signal, not a mixdown of other channels.

Even the Oppo disc player fits into this relatively simple setup. Multichannel content goes directly to the Marantz via HDMI. Hirez DSD & PCM stereo goes directly to the T+A, via the Oppo’s second HDMI output -- thus, no HDCP constraints when playing SACD hirez stereo.

Of course, the proof is in the pudding, and it took a bit of fiddling and room-correction to nail down crossover frequencies & channel levels that make this all work. But it’s sounding mighty good right now.

The only thing I couldn't do is bypass the Marantz DAC & preamp for FL/FR output when playing multichannel.  But that doesn't seem to be possible without some significant compromises in functionality.  Oh well.

Regardless, I think we can safely say: "Problem solved."