The two preamp circuits in the chain is how some reviewers tested the setup:
https://www.stereophile.com/content/ps-audio-bhk-signature-preamplifier-page-2
https://www.stereophile.com/content/ps-audio-bhk-signature-preamplifier-page-2
Are these reviewers hearing the coloration from the preamp that makes the music more enjoyable?
What I said above is that the amplifiers used in those do
not have a preamplifier, unlike your Studio Six. They used mono amplifiers, which, if each of them had its own preamp, would need to be adjusted twice over since each amplifier for each side will have its independent volume control.
In that review, the other preamp is on the DAC, which has a digital output with a digital volume control. What they did is practically like hooking up a computer to a Schiit DAC and headphone amp+preamp, or a Schiit DAC and passive preamp, then used Windows volume control. Except the PSAudio DAC they used likely doesn't affecct bit length, unlike Windows before Win10.
Still arguably unnecessary since they're still going to use the DAC's volume control anyway because they want to use a remote,
so they're not even really using the preamp anyway. That said, it might not necessarily just be outright coloration, but much higher gain. The output of the DAC is a lot lower, AFAIK max of 4volts at full tilt from the balanced output. While that adds gain, and easy to assume that it does so without coloring the sound badly (much less add noise), if I was going to add the preamp I'd just leave the DAC to output at full line voltage and use the preamp. I mean, how many times exactly do I need to get up and change the volume? You do that with headphones because sound sources from a computer can vary (ie YouTube channels each have different volume levels vs locally stored and Spotify to which you can apply replay gain), not to mention swapping headphones and they have differnet impedance and sensitivity (and frequency response, which can affect your perception of how loud they are at 1000hz), but if you're in a dedicated listening set up using only locally stored digital music on speakers, changing the volume setting frequently is a lot less necessary. In the end, they're still using what amounts to a passive preamp on the DAC, so it's like using a Schiit Sys to control a headphone amp or integrated amp, or a preamp controlling a power amp.
In your case it's the amp that has the preamp, not just a variable output on the DAC, so technically what you will end up with is
two active preamps in that chain increasing the gain twice over.
