Sorry for the delay in answering your question.
I went from a ARC REF 75 to the D400.
The D400 has much better low end control, but I miss the low level detail and 3D images I had with the tubes.
The REF 75 had no problem driving my WA S3s with their 87 db sensitiviy, so the new REF 75SE or GSi75 should drive your speakers.