Also there's a lot of radio promotion scheduled for Kuu:
http://rhythmzone.net/koda/schedule/...page=1&a=media
Releases on the same day like Kuu:
AFTERSCHOOL's Best
U-KISS
Idoling!!
KOKIA
But I guess only Idoling!! could end up being a threat.
Though there are quite a lot older solo artists releasing new albums as well, but I just don't know, who's popular.