Hi,
Starting with side note, Pocket MIDI cannot probably open the LPD8 port because you had Cubase open. I have LPD8 as well and it does the same - Pocket MIDI has to be the only application trying to read the port of a MIDI device.
So did I understand correctly that if you select SLMIXUSB in Pocket MIDI you can see MIDI information if you move faders or turn knobs but nothing happens when pushing transport buttons like play/stop etc.? Actually, that is normal behaviour in case you are in CONTROLLER mode of Mixface, because Mixface as 3 different MIDI ports (as listed by Pocket MIDI):
SLMIXUSB = Midi port 1, which is for data fader/knob coming from Mixface when it is in CONTROLLER mode.
MIDIIN3 (SLMIXUSB) = Midi port 3, which is data coming from Mixface when it is in DAW mode - also transport button data ALWAYS comes through this port!
(…and MIDIIN2 (SLMIXUSB) which is used for firmware updates but have no other use)
So, when you have selected SLMIXUSB in Pocket MIDI and you are in CONTROL mode, you only see MIDI messages from faders and knobs, like you saw. If you press eg. play button, its output is always going to Port 3, not the port “SLMIXUSB” you are reading. Now, what you have to do is to select MIDIIN3 (SLMIXUSB) in Pocket MIDI. If you do that, you should see messages like I posted above no matter whether you are in CONTROL or DAW mode, because Mixface always transmits those buttons through its port 3.
Still, this does not explain why it does not work in Cubase - unless you have there as well selected SLMIXUSB instead of the “MIDIIN3 (SLMIXUSB)” in your Studio Setup menu’s Mackie Control device?