Update - thanks to everyone who chimed in answers! I bought an RME ADI-2 DAC fs, which has a headphone amp with 2 outlets, one for IEM’s. I also bought the Sennheiser HD600’s. They sound great together. This gives me the option to listen late at night without having to turn down the volume very low.
I chose this to avoid having another box and wires, as well as giving me a wide variety of controls including parametric EQ. I also bought a UMIK 1, so I can do some room testing and perhaps adjust the sound for further improvements. Since I don’t have a preamp, I had no adjustments before. Enjoying the upgrade!