The differences are as follows.
Stereo Summing Mixer with Drive (6) is 4 x stereo input channels to 1 stereo output. No bypass available.
Stereo Summing Mixer with Drive (7) is 4 x stereo input channels to 1 stereo output. There is a bypass control that patches the 8 inputs straight to the 8 outputs.
Stereo Summing Mixer with Drive PRO allows either 8 x mono input channels or 4 x stereo input channels. Each channel has a pan (in mono) or a balance (in stereo mode). Bypass has 3 options. 1) Channels are mixed down to stereo pair 1/2. 2) The 8 outputs have the drive applied only, making this an 8 channel saturator, in either stereo or mono mode. 3) The 8 inputs are connected to the 8 output for true bypass. This algorithm has a rearranged UI with 1 page per channel.
I haven’t tested the Pro version extensively yet, so if you find something that doesn’t work as expected, please let me know. The wiring is very complicated for this one, and there is a possibility that I got 1 or 2 wires mixed up. I’ll do some presets for this one shortly as well, to provide some useful starting points. If you have some favourite presets, feel free to send them through and I’ll include them in the package.
Hope this helps.