I'm not 100% sure what you mean, but if you're talking about using the Darkvoice's Preamp output and then feed the NAD 7155 with that, and why the headphone out is louder than the Preamp out, then I think it's due to the resistors placed in the final preamp out stage where the coupling caps are (Section with .47uf caps and 1mohm resistors, the 10kohm ones on that board tho are to ground and I don't think those are in the way of the actual signal, it's been a while since I modded my Darkvoice so I don't remember). I think they just added resistors to make the preamp stage not so loud or maybe so it mixes better with other systems.
Also remember, resistance isn't linear in how much it drops current. 12 v from 1 to 2 ohms drops WAY more current respectively than 1000 ohm to 1001 ohm, and that may also affect how much the potentiometer is actually doing resistance wise (it's 250kohm I believe), so maybe there's a point where the preout's resistance simply makes the potentiometer's resistance pointless for that section?
Honestly I just removed that all together and made the Darkvoice be a preamp from the headphone output, as well as a normal headphone amp, and it's damn quiet after mods, but keeping that "Tube Sound" everyone loves.
Also preamp output adds loading, and I think it's because the resistors act as a load in that section, and since there isn't a switch telling the amp to either be a headphone out or a preamp out, then they both are functioning at the same time to an extent, like having two headphones consuming from the same signal at the same time almost with no circuit to compensate for that.
I mean you could remove those resistors and maybe that'll make the preamp louder as well as make the potentiometer affect the loudness of the audio past 9 o'clock. If you do that just be wary of the power supply capacitors since even though they SHOULD be drained to safe levels where touching them shouldn't do anything, it may not, so have a multi meter on hand to test the caps, since the amp doesn't have a bleed resistor there so the inserted tubes tend to drain the caps when you turn off the amp, altho not completely, leaving 10 volts, that while safe, isn't assured, so just test it if you do go this route.