Being a 33DRK/WAR, I would like to say that solo'ing as DRK/WAR alone is more tough than WAR/MNK due to the low defense. Going DRK/RDM would probably further hinder you without adding too many benefits. It would give a minor magic boost, but in all honesty, DRKs use a whole lot less magic than you think, which is why DRK/BLM is less accepted than a melee sub.
Subbing WHM isn't exactly too great either, it may help you solo a bit more, but DRKs can naturally heal themselves with a Drain - Aspir combo. BLM would increase the effectiveness of your offensive spells, but they would never be up to par with a BLM/WHM or a RDM, and it would nerf your melee potential.