This is a bit incorrect.
The SDR mod strives more for neutrality.
It is NOT similar to the 800S.
It is more similar to an 800 with less trebles
The 800S is having a decidedly increased lower spectrum, although not as "clean".
So, If you want the best of both worlds, (that being an 800S with the cleanness of the 800), you would install the Sennheiser foam mod, which I posted as part of my overall modification, which I have listed in my sig..
About the vocals, the midrange becomes better or more prominent on the higher impedance tube amps, and on speaker amps.
In both cases the HD800 is probably getting the juice it needs, because its impedance, at 600ohm is actually actually an average, so the demands vary with frequency.
Although theoretically, this shouldn't be a problem for an amp, in reality "suppy-n-demand" is not linear.
The Various response of an amps power supply, or "power-on-tap" reserves will make an amp sound different to the HD800.
The HD800S does seem to alleviate the situation with more mids, for those that prefer it.