You did not mention the size of the room but it may be quite small given your choice for these small speakers in a high quality system. My minimum recommendation in a very good system such as yours would be two small quality subs such as the SVS SB1000 supplemented by some room eq. Since you have the Pro version of the RME DAC you can probably set a high pass filter on the balanced output to the power amp. Use the remaining filters on that output to equalize the range above the high pass filter and up to perhaps 250 Hz. You can then use the separate filters on the unbalanced output to equalize the subs.
If the room is larger and offers more space for subs I would add a third, perhaps larger one. In that situation MSO also becomes more relevant.
If the room is larger and offers more space for subs I would add a third, perhaps larger one. In that situation MSO also becomes more relevant.