Sure but is it $600 better?
I think thats an individual choice. For me, I love it. I have a bunch of IEMs and Headphones and the ability to EQ and SAVE each one is great. I can also EQ my speakers. The ability to fine tune every setting like cross-feed and balance and everything is quite nice too. In general, the DAC on the RME is more detailed than the DAC on the Monolith, despite it using an older 4490 vs 4493 DAC. The output on the RME for IEMs is really well done and black as midnight on a moonless night. The Monolith THX has audible noise when I use some sensitive IEMs.
The headphone output on the RME is quite sufficient on high gain, while the THX does have better output in general using balanced, but I am perfectly fine using the 1/4" out on the RME right now. I switch off and on between both (with RME being the DAC for the Monolith).
Other than that, having a remote is a nice feature. I honestly like both of them, but I like the RME more because it has more features and sounds better to me.