You already tried your solution:
" Second, when I connected a PS Audio DSJr through a Marantz AV receiver as preamp, then to the exact same Musical Fidelity amp and Monitor Audio speakers, the sound was GLORIOUS in comparison to the NAD"
You don't need amplification so a Pre-Processor would be ideal. Marantz Pre-Pro are readily available, both new and used. 7705 from Audio Advisor: $2199 NIB FYI- Kalman R was very surprised by the quality of the DAC in the next model up...8805, stating that it took upwards of $5k in external DACs to improve digital reproduction.
https://www.audioadvisor.com/prodinfo.asp?number=MAAV7705
" Second, when I connected a PS Audio DSJr through a Marantz AV receiver as preamp, then to the exact same Musical Fidelity amp and Monitor Audio speakers, the sound was GLORIOUS in comparison to the NAD"
You don't need amplification so a Pre-Processor would be ideal. Marantz Pre-Pro are readily available, both new and used. 7705 from Audio Advisor: $2199 NIB FYI- Kalman R was very surprised by the quality of the DAC in the next model up...8805, stating that it took upwards of $5k in external DACs to improve digital reproduction.
https://www.audioadvisor.com/prodinfo.asp?number=MAAV7705