The output of the DirectStream DAC is transformer coupled, and in that sense can loosely be considered to be passive, but unlike resistive-based passive preamps it has an output impedance that is low enough to be comparable to that of most active preamps, both tube and solid state. That is confirmed in
John Atkinson's measurements.
Given that, as well as the 40K balanced input impedance of your amp, as well as PS Audio's claim that its volume control mechanism does not lose resolution at any setting (which if it occurred at all would occur at low settings of the control, not high settings), I see no technical reason that would call for the insertion of a preamp. Which is not to rule out the possibility that you might find adding a preamp to be subjectively preferable. But FWIW my own bias is to put the burden of proof on adding anything to the signal path that is not clearly necessary, and in this case I don't see it as being clearly or technically necessary.
The reason for the relatively high settings of the volume control you are using are most likely the combination of the "full scale" (maximum) output voltage of the DAC being a bit lower than usual, and the sensitivity of your speakers being somewhat low (Stereophile measured the speakers as 86.6 db/2.83 volts/1 meter, which for their 4 ohm nominal impedance is 83.6 db/1 watt/1 meter).
So as long as you never find yourself wanting to turn the volume control up beyond the top of its range, I wouldn't attach any significance to the fact that you are using it in the upper part of its range.
A couple of things to check, though:
1)Apparently the DAC's output can be set to two different levels via the menus. Check to see that it is set for the high level, i.e., that the output attenuator that is provided is deselected.
2)Check that the unit has the latest firmware (version 1.2.1) installed. User comments as well as PS Audio indicate that that update tightens up and improves the bass. I would expect that you can determine the firmware version somewhere in the menus, or perhaps it is displayed briefly at startup. If it is an earlier version, you can easily download and install that version per the instructions at PS Audio's site.
Regarding the bass traps, it's of course sometimes possible to have too much absorption in a room as well as too little absorption, especially from a subjective standpoint, and depending on the speakers, their placement, etc. I suspect that the main reason for the more powerful bass reported in your latest posts was removal of the traps from the front corners, rather than their placement behind the listening position, in part because the change was perceivable at other than the listening position. Hopefully after doing the remaining experiment (removing the traps from the room altogether), and some further listening, you'll be able to determine which of the four possibilities is preferable (traps or no traps in front; traps or no traps in the rear).
Continued good luck. Regards,
-- Al
