Heres a different approach you can try ... your Krell KCT has a number of premium low level MAIN outputs you can put to good use
You can run a XLR balanced cable from KCT MAIN OUT (#22 in the manual) to the INPUT of 400CX amp ... then run speaker wires from the 400CX to B&Ws ... this keeps the Sub out of Premium Krell/B&W loop
As your powered Sub most likely has a low level RCA IN ... run a normal single ended RCA (#22 in the manual) from KCT MAIN OUT to the SUB INPUT RCA input on powered SUB
Now the 400CX and B&W dont see the Subs lower quality electronics and the 400CX isnt trying to drive the B&Ws and the SUB at the same time
Each is running exclusively of each other ... but when you turn the volume up or down on the KCT Pre amp it will turn the volume up or down equally on both the SUB and 400CX amp
You can also use the special CAST Output (#23 in the manual) to drive the CAST Inputs on the 400CX also and still use the RCA MAIN OUT to drive the SUBs input
CAST connection requires special Krell CAST Cabling ...
Possibly you can tell us what brand and model the Sub is and how you connect the KCT to the 400CX
You can run a XLR balanced cable from KCT MAIN OUT (#22 in the manual) to the INPUT of 400CX amp ... then run speaker wires from the 400CX to B&Ws ... this keeps the Sub out of Premium Krell/B&W loop
As your powered Sub most likely has a low level RCA IN ... run a normal single ended RCA (#22 in the manual) from KCT MAIN OUT to the SUB INPUT RCA input on powered SUB
Now the 400CX and B&W dont see the Subs lower quality electronics and the 400CX isnt trying to drive the B&Ws and the SUB at the same time
Each is running exclusively of each other ... but when you turn the volume up or down on the KCT Pre amp it will turn the volume up or down equally on both the SUB and 400CX amp
You can also use the special CAST Output (#23 in the manual) to drive the CAST Inputs on the 400CX also and still use the RCA MAIN OUT to drive the SUBs input
CAST connection requires special Krell CAST Cabling ...
Possibly you can tell us what brand and model the Sub is and how you connect the KCT to the 400CX