As a sub I think cleric is the only way to go.
Bard would be your main mastery (as battlebard) and you will benefit the most from cleric with skills like bless spell, recovery division and holy spell.
If you would take wizard or warlock, they would be your main mastery (attack skills) and bard more your sub (the buffs).
Unless you want to be able to play as a battlebard one time, and the othertime as a wizard or warlock. But i dont think its a sub anymore that way, just a 2nd equal mastery. Will be really sp costly.
This is just my opinion
