You have pioneer 6999's cool cus i have th very same speaker, what do you think of them, rather nice sound i think.
Sorry i just have never come across any one with the 6999's before.
as for your questions:
1. If you ground every thing (to the same common ground), there shouldn't bee too much noise, if there is a noticable amount of noise, just a ground loop isolator (from radioshack)
2. Subs shouldn't really kill your copmputer, just I would tend to keep the hdd away a little, just to be on the safe side.
3.To power the unit, u will need either an Inverter, (dc-ac converter) or a dc-dc converter (better, will also help elimiate noise)
Just read the forums, and use the search function, and you will find that all these topics have been discussed previously