this will work very well. either the flip top or bassman would be good starting points, or perhaps just using the "tube preamp" depending on your taste.
1) you can control the HD Pro/X via MIDI, so any MIDI controller set the the same MIDI channel will work.
2) The MIDI PC/CC control is typically limited by the controller. I have the original Ground control unit and that sends 8 MIDI channel messages simultaniously... and it is probably 10-15 years old so I am sure newer units can do more.
3) As the POD HD units also have bass modeling (Ampeg and Bassman) I am betting it will be able to handle low end from most applications. Frequency response tends to be more in the speakers you are using.
4) the tuner works fine for live applications and demo work IMO. I have measured the tuner to be +/- 2 cents compared to my Peterson tuners, which is the same as most "standard" tuners available on the market for ~$100 USD.
See the link for more info: http://line6.com/podhdprox/
AndyParedesMusic's post in Bass>podhd500x>l2t was marked as the answer