Thanks for all the advice!
I'd like to clear up a mistake I made in my first post - there's actually 120W of light on the tank, not 80.
I think I've discovered my first problem : the SAE's I bought are actually Siamese Flying Foxes. Apparently they're mislabelled very often; and only "true" Siamese algae eaters will consume the black brush algae. I've contacted every LFS within 3 hours of me but none have any true SAE's in stock.
So far I've removed the heavily infested leaves from the tank altogether and have done two rather thorough wire-brush scrapings of all the rock and aquarium equipment in the tank. Two hefty gravel vacuumings were done as well to try to clear up all the loose BBA floating around. It looks quite a bit better now, but I'm sure it will come back soon enough - all I've done so far is cosmetic work.
Tomorrow I'm going to pick up a phosphate test kit to see what's going on. As for pH, I'm sort of stuck keeping it in the high 7 range, since it's an African tank. Ammonia, nitrates and nitrites are all 0; kH is 11.
Does anyone know why I would be having this problem? The tank is really quite sparsely planted; as it's an African tank it's mostly decorated with rocks and I've only got 3 plants in the whole tank.
1x 5" Gephyrochromis acei
3x 3" Yellow labs
2x 4" Siamese algae eater