Yeah, there's always gonna be some headphone that doesn't work well with some amp. As for the RCA inputs being too loud, it depends on the source's output voltage, and what I recall trying with the EF2 are the Pico DAC-only, iBasso D10 and D4, andNuforce uDAC's which weren't too loud.
So is there any amp you recommend for IEMs. Let's say with the budget under $200. Will the amp improve the sound or just a good DAC is enough?