Q:

I'd like to route the Traktor Scratch Pro FX engines as sends, controlled via the XONE:4D's FX SEND/FX RETURN pots so that the effects continue even when the Traktor DECK, or XONE:4D Mixer Channel, faders are down (i.e. as a pre-fade send/return, not as an insert).

Is this possible with the soundcard configuration switches, or some extra cabling? 

A: 

Yes, by using the XONE:4D's physical SOUNDCARD OUTPUT connections and a Stereo RCA cable. Configuration is as follows: 

1. In TRAKTOR, set the Effect Unit Routing:
   Go to: PREFERENCES > EFFECTS
   Change the FX1, or FX2 Effect Unit Routing as required, from the default Insert setting to Send.
   Note: The FX ON switch in the ‘Filter + Key + Gain + Cue + Balance’ section of TRAKTOR’s DECKs will not be visible when FX Units are set to Send mode.
   
   
2. In TRAKTOR, set the Input Send:
   Go to: PREFERENCES > INPUT ROUTING
   Select either the XONE:4D SC3-4 (FX2) or SC5-6 (FX1) soundcard inputs as the TRAKTOR Input Send Channel.
   
   
3. On the XONE:4D, set the Soundcard Input Selection Switch: To enable signals to be routed to TRAKTOR’s Input Send Channel from any XONE:4D mixer channel, set the corresponding Soundcard Input Selection Switch to the special feature:
   If SC3-4 are selected as TRAKTOR’s Input Sends, set the XONE:4D CH2 Soundcard Input Selection Switch to FX2 [UP].
   If SC5-6 are selected as TRAKTOR’s Input Sends, set the XONE:4D CH3 Soundcard Input Selection Switch to FX1 [UP].
   The FX1 or FX2 SEND controls are used to adjust the amount of audio signal routed from the mixer channel to the TRAKTOR FX Unit.
   FX1 can be configured to PRE, or POST channel fader level via the top panel switch, while FX2 is fixed as a PRE-FADE send.
   
   
4.  In TRAKTOR, set the Output Send: Go to PREFERENCES > OUTPUT ROUTING
   Select one of the XONE:4D stereo SOUNDCARD OUTPUTs (1-2, 3-4, or 5-6) as the TRAKTOR Output Send channels.
   
   
5.  On the XONE:4D, connect SOUNDCARD OUTPUTs to the FX RTN bus: There is a physical, RCA, connection available on the back of the mixer for each XONE:4D SOUNDCARD OUTPUT.
   Connect a Stereo RCA cable from the XONE:4D SOUNDCARD OUTPUTs selected in TRAKTOR’s Output Send channel (3. above), to the FX1, or FX2 L/R RETURNS on the back of the XONE:4D.
   Use the corresponding FX1 RTN or FX2 RTN control to adjust the wet (return) signal level from the TRAKTOR FX Unit set to SEND.
   Note: Because a soundcard bus can only be assigned to one TRAKTOR Channel at a time, this configuration means one stereo SOUNDCARD INPUT bus and one stereo SOUNDCARD OUTPUT bus, of the four A-to-D/D-to-A buses available from the XONE:4D, cannot be used simultaneously as Input, or Output Channels for TRAKTOR DECKs.

Q:

OK, but it’s a pity 9/10 can’t be used for this.... 

A:

XONE:4D soundcard output channels 9/10 are digital signal outputs (SPDIF, or OPTICAL), so must be connected to a device that can accept a digital signal input. Both XONE:4D FX RETURN buses are analogue connections.

However, the XONE:4D's digital soundcard bus could be used to this end, by connecting an external audio DAC (Digital-to-Analogue Converter), in-line between the XONE:4D SPDIF (or OPTICAL) OUTPUT and FX RETURN connectors, and setting the XONE:4D SC9/10 outputs as the Output Send channel source for TRAKTOR (see 3. above). 

A host of DACs are available on the market; with only a brief internet search we found many, with prices ranging from @ €50-60 to many hundreds.