RCA connection between mixer and headphone amp would also work, yes.
If you want an all-in-one like the AG03, the options are limited:
- Behringer Xenyx Q502 (Behringer, so quality is meh)
- ART USB Mix (no experience with it, ART has been good to me though)manual is not clear if all 3 inputs get routed to USB# - ART USBMix4
- Mackie BigKnob Studio
- Soundcraft Notepad-5 (reviews are mixed, so take this as a “last resort”-option)
Or you get an interface (cheap one: Behringer UMC22, medium: Scarlett Solo) and connect that and your E30 to a mixer (Alto ZMX52, Mackie Mix5, etc.) wich then feeds the 789.
You would end up with a setup like I have