Grrrr! I wrote a long, full response and that response was lost. I must remember to stay away from Zynthian Discourse in the morning when it is still waking up, stretching its arms and before its first coffee. (I believe Discourse is sluggish because its host server is busy building nightlies.)
Anyway, the gist of what I wrote is: We should consider use cases. Balanced (differential) inputs help to reject common mode interference on long cable runs, electrically noisy environments or low signal levels. The signal is soon converted back to single reference so there is limited use of using a DI + balanced inputs if the unbalanced inputs are available. Impedance matching is an important element in reducing hum, buzz, noise, etc. Balanced inputs are traditionally lower impedance than unbalanced inputs. Guitars expect a high impedance input whilst microphones often expect a low impedance input. I don’t really understand the design principals used for the HB input. It does not align with traditional levels / impedance / differential input / etc. so I find it difficult to advice how best to interface with it.
A DI can provide impedance and level matching but this may alreadty be available with the inputs of the HB. If the HB accepted balanced inputs with varying gain / attenuation then a DI would make sense although a switch might be as good, changing the input configuration. Tricky to say without playing with one. It is a little too much for me to justify buying. I really like the idea of audio input to the Zynth but want to implement the most flexible solution without spending too much.
(I feel frustrated that this response may be missing elements from my lost response but I do need to work so here it is!)