I have finished connecting my Flex 6300 up with WJTX-X v1.70 application in WSPR mode. Dudley and Bill (G4WJS) helped making it work since I had trouble with the Receive and Transmit Hopping feature switching to LSB. All bands use USB for WSPR operation. Below is a photo (busy) but is a full description of my working WSPR mode. 73s, Jerry WA0RCX
I've looked for you on the WSPRnet and have not seeing you there yet...
Band conditions on 20 seems pretty slim today: mostly European stations.
I had to call my buddy KF2T in Nevada to get it setup.
running less than 2 Watts to a Flex-6700 into a Hy-Gain 18HT vertical in IOTA NA-138 Amelia