Oops, forgot to answer your question as to balanced or not. I've experimented with quite a few headphone amps as you know and quite a few headphones
I would not be so concerned about going balanced since the topology and implementation of balanced varies from amp to amp. Some amps that offer both will sound significantly better, some not so much. Some Single Ended will blow other balanced out of the water so it all depends. No, I would not be looking for balanced specifically unless the amp is clearly known to perform better in balanced mode, such as the Bryston and Violectric for example. The SPL Phonitor on the other hand only offered single ended but with 60v rails, you'd be hard pressed to ever hear a difference in the drive and overall grip on ANY headphone of any impedance between those amps.