This is going to be messy. Firstly, a HDMI audio stripper isn't the answer. The signal level will still be fixed at line level because anything capable of carrying a Bitstream signal (DD, DTS, HD audio) isn't allowed to have the audio level varied.
I understand how your music system works. The DAC is doing the job of a being the pre-amp and volume control that drives the 2ch power amp. However, in linking it to integrate the front channel pre-outs of the Onkyo, the DA-1000 is no longer the volume control, that role is taken over by the Onkyo. Since there is no analogue input on the DA-1000, the analogue FR+FL pre-out signal needs to be converted to a digital signal to be compatible with the DAC. That means changing to signal level in the digital domain, and that's bad within the context of a system with the resolution of yours.
The reason it's a bad idea is to do with a loss of resolution that will result.
If we said that the sampling window for a digital signal had size of 100dB at line level, then that represents a lot of individual steps of change. The more steps then the greater the accuracy of the conversion. Within that window there'll be the noise floor, and also the dynamic range of the signal itself which could be 70 to 90dB for TV and movies.
Making the signal level variable in the digital domain means changing the size of the sampling window to correspond with any volume changes. The signal contained in the sampling window still has the same dynamic range (say 90dB), but it's being compressed in to a smaller and smaller space as the volume is reduced. A crude analogy would be what happens with colour pictures when the colour depth is reduced.
A 24bit colour RGB image has 16.7 million colour shades available. The red, the green and the blue channels each have an 8 bit sample size, and this gives a possible shade range of 256 levels per channel. To all intents and purposes it looks like a full colour image on screen. However, make a small change in the sample window to 16bit colour and the result is a vastly reduce number of colours. The number drops from 16.7 million to just 65,500. Definite colour bands are visible and there's a lack of smooth shade changes. It all gets a bit painting-by-numbers. The smaller the sample window then the more pronounced the effect until tit gets down to just two shades on screen. The same happens with the audio.
The best resolution is where the signal is playing straight through with no attenuation, but at this level it's blasting out as if you've just connected a CD player direct to the Bryston with no volume control at all. Reducing the level of the volume coarsens the sampling. I'm not even going to get in to dithering.
The right way to make this work is with either an analogue audio switch or a passive pre-amp between the DA-1000 and the Bryston. The problem you'll have is finding something transparent enough that it doesn't change the signal when playing music, and maybe it has to have balanced connections too. A £12,000 DAC in to a £5,000 stereo power amp and going to a £4,000 set of speakers is going to be a fairly revealing system of anything that's sub-par.
Have you got a budget in mind?
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