the big Octabank works best with more power than an AB delivers. Also, it's a pain to use the Octabank it with a monblock.
For fill 400 watts is enough. As a key or a single light, more wattage the better.
It should be OK with a monolight, as long as it is fully remote control. Having to open the Otcta up everytime you needed to change a setting would be an enormous pain.
I used to love my Octa back in 2005. Then I started using Briese Focus stuff, and the big Brons as key lights, and the Octas less and less, and then only for fill. This is really a matter of taste and style, but the Octa, even with the difussion and baffle removed, was never directional enough for me. I could do some grip work to get it close to the focus and the Bron Paras, usually involving several lights.