Originally Posted by Omega
I use a Beresford 7510 MarkV and an RSA Hornet (original non-M version), with both Shure E4c and Beyer DT880 headphones. The Hornet absolutely slays the builtin headphone amp in the Beresford when using the higher impedance (and much more capable) DT880's. The Hornet is better in every way. Better attack, better bass, less congestion, louder clean volumes, ...
I've not had experience with many other headphone amps, but I'd call the built-in Beresford headphone amp decent at best. The DAC is quite good, it deserves to play through a good amp!
I'd like to second this. I should have the mk5 (bought mine maybe a month or two ago before the mk6 came). I have it hooked up to a heed canamp. and when comparing the headphone out on the two, there is a striking difference in the bass. The mk5 seems bloaty and loud, whereas the canamp's bass is tight and controlled. The mk5's bass in my opinion adds to the congestion because of its great bass quantity (with the lack of quality). Whereas the canamp's bass is has the quality and in my opinion, because of the quality, it's less in quantity than the mk5.
Other more minute differences I cannot distinguish, because I've not made anymore attempts to use the mk5's headphone out.