The D-2.1’s have a stated sensitivity of 89Db at 2.83V and impedance >4ohms.
As far as I know the Diablo Integrated amplifier is made in two configurations;
The Diablo (300) is rated at 300w at 8ohms and ~600w at 4ohms.
The Diablo (120) is rated at 120w at 8ohms and ~240w at 4ohms.
either Diablo should adequately drive these speakers. The recommended amplification is for amps with 50w/ch and higher but can be driven by lower powered amps.
Check to see if you have the DS attenuator switched on.
Is DS DAC volume set to 100?
Are you using XLR or RCA connection between DAC and Diablo (shouldn’t matter).
Did/Do you have a problem driving the speakers with other inputs or have you only tried with the PWT/DSD DAC?
Nice system by the way.
When you say the amplifier doesn’t drive the speakers well - do you mean volume? Can you measure volume in Db with a smartphone or meter? What is volume dial on amplifier at 89Db at 1 metrr from the speaker?