You'll be fine with topsoil and compost mixed at roughly 50/50. When I build planter boxes for bamboo that's the soil mix I use. If it were a smaller planter or pot then I'd make sure that it had more peat and/or perlite to keep it from compacting, but the bigger planters don't seem to have that problem. Bamboo rhizomes and roots don't seem to mind heavy soil.

Bamboo do well in a wide variety of soils. Those I have planted in my yard are in hard, somewhat clay-like soil, and I don't bother to improve it. For those I put in containers, I pick up a load of standard top soil from my local yard materials place. As long as your water drains adequately (i.e., you don't have standing water) you should be fine. I do sometimes throw a handful of slow release fertilizer in the hole before planting the root ball, but that's it.

Thanks for the quick responses. I was starting to get the feeling I was over-thinking this. I figure bamboo is pretty hardy and seems to tolerate a variety of soil conditions. I think I'll go the 50/50 topsoil/compost route and just ensure there is adequate drainage in the planter box.