Drop just came out with an unbalanced version of the THX 789 (called DROP + THX AAA ONE) for 149$, wich doesn't seem too bad either. Might be an option to consider if it makes sense logistically.
Balanced would be worth it if you are going to use active monitors or any other components, that lead to more than one grounded (recognizable by the three prong mains plug) unit being electrically connected (like USB, RCA, Aux etc., but not Toslink) in one circuit.
A reasonably high powered single-ended headphone amp with low output impedance and otherwise decent mesured performance should absolutely do the trick, you don't need to spend a lot of money necessarily.
The 1990 Pro in particular isn't all that hard to drive despite it's 250 Ohm impedance due to fairly high sensitivity, any of the usual recommendations (JDS Atom amp, Schiit Magni Heresy, Archel2 Pro, Topping L30 [when it comes out] etc.) easily does the trick.
As far as the DAC is concerned, there are a bunch of good options as well. For me, there wasn‘t really an audible difference between the Apple dongle and my current Topping E30 - just as the measurements would suggest, both are audibly transparent. For me, the features make all the difference between all these well performing low cost desktop USB DACs.
Especially when using an amp with analog volume control (and not <1x gain) a DAC with volume control can be very much worth it, since you can lower the volume below what the analog pot would usefully allow without losing (markedly) dynamic or destroying the channel balance.