Will the third as of yet not named amplifier have more power than 789, is this a headphone amp and sensitive speaker amp?
Either way id say give it a headphone socket in the same way emotiva do on the Basx100 to allow the full power of the amplifier to be able to drive planars properly and other hard to drive headphones.
Oh oh and make a tone control like the schiit Loki to match the drop stack.