I was using my $1200 German made Lehmann Audio Black Cube Linear amp (a neutral amp), on the MIXR & BeatsPro I got them to play pretty loud (beyond what I would listen at) with the volume knob at 12' o'clock. On the Solo HD the volume knob was near 4 o'clock to get the same volume level. I used the amp because I found the Solo HD to sound dull and muffled on my iPhone 4s. Playing the same tracks through the Lehmann amp, the music came alive. The bass on the Solo HD was hitting so hard my ear & lobs were vibrating. I wanted to play it even louder, but I know I shouldn't due to the risk of hear damage. The Solo HD did not distort at all at any levels when using my amp. It was an eye opener so say the least.
Holy smokes !! 4 o'clock is pretty damn loud..I never get past 11 o'clock on my HD800 using S7. Beats solo must be one hell of a inefficient headphone!!