The volume differences are do to the drawbars settings used in the preset values. 888000000 will be twice as loud as 444000000. Each one of the presets has varying degrees of settings. Gospel settings like 800000008 will have a much quieter volume than 888888888. There is no volume adjustment to get them all to be the same other than to increasing/decreasing the volume knob on the XK1 or increasing/decreasing the expression pedal (if you are using one). This is how a hammond works and the XK1 emulates that.
If you feel the overall output of the instrument is inadequate even with the volume knob on the XK1 set to maximum then I would put a mixer in between the XK1 and the amplifier that you are using. I have to do this with my ELX112P. The speaker needs an input boost in order for it to be driven to the proper level. I use an Rolls MX28, only slightly bigger than a cigarette pack but provides a 20db gain on the signal which the ELX112P needs in order to be loud enough in a band situation.