The hard treadling when plying is I believe due to the brake band gripping on the bobbin whorl. To overcome this there are a number of solutions:
1) Smooth the bottom of the bobbin whorl groove with sandpaper or scotchbrite.
2) Apply a drop of oil to the bobbin whorl to allow the brake band to slip more easily.
3) On the Joy we drill pilot holes beside the holes for brake tension knob and threading hook knob. These pilot holes allow you to swap the screw eye attached to the tension spring from one side to the other as well as the tension knob and threading hook. Have the spring on the left when spinning (clockwise) and on the right when plying (anticlockwise). The tension spring lets the brake band slip which makes treadling easier.
4) Cut the brake band between the tension knob and bobbin whorl and add a second tension spring. This means there is no need to swap the tension knob from side to side. Springs are available from Ashford dealers all over the USA.