Looking for a way to enlarge the soundstage of my HE400(92.5dB/35 Ohms) I try it direct no volume control in the 2V RCA outputs from the CD player Sony SCD1 from 1999, to my surprise the sound was quite loud even w/the balmy smooth Sérénade Mélancolique(Maazel-Kremer) and plenty of new details before unheard.
With pop music as Art of Noise-Box Set Sampler the sound was extreme loud, after afew tracks I get ear fatigue!
So I confirm you dont need headphone amp to drive most headphones, instead you will need a potentiometer to down volume control, I suspect the ideal sound level to listen direct at the RCA outputs with no volume control will be a 90dB headphone as the HE560-45 Ohms, but if you has a CD player with volume control dont sell it.
I will listen other CDs and SACD when time allow to report.
Unfortunately the HE400 soundstage dint expand more than before when feed direct with the RCA outputs, the soundstage had the same size as the PC board with radio and YT music.
Tschaikowsky: Violin Konzert - Gidon Kremer, Berliner Phil-Maazel DG 400.027-2 1980
Art of Noise: Box Set Sampler ZTT202CD, 2006