I've been wondering the same thing. I just got a Marantz SR5011 receiver and the Focal Elear headphones. I absolutely love the headphones, but at the store where I tried them a Marantz CD player and Beyerdynamic amp were used. With that setup I remember the headphones sounding more focused and a little bit clearer than on my system. Honestly the difference is probably only 1-2% and I'm still amazed at what the Elears can do every time I put them on.
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I'm also wondering if maybe the perceived tiny difference between my setup and the store's is entirely in my head. In the 1 week that I've had the Elears I've noticed that when I find the correct volume it just "clicks".
That has a lot to do with the 104dB/1mW sensitivity of the Elear, ie, they don't need a lot of power. Using something like a Schiit Lyr for example on the Elear is kind of like using a B.A.T. dual mono amplifier on a horn speaker that is otherwise used with flea amps.
That said, the Beyerdynamic amp has a high output impedance, and likely the Marantz also. So
if you ever try an amp, you don't need one with a lot of power, so just get anything with a low output impedance and low noise, like the Corda Rock. This actually still has a lot more power than what the Elear needs, but you'll get that with low noise and for not a lot of money. For even less money there's the JDS Labs O2.
Note though that you might like the sound of the Meiers and the O2 a lot less since the low output impedance also removes the EQ effectthat typically can either boost or trim the low end on low impedance headphones (and boost the midrange on high imepdance headphones).
I'm not sure why the Sennheiser 598 sounded so bad in the Marantz, however.
For its price, the Elear should very well sound better off the bat than the HD598. So just in their natural response, assuming the amp isn't having any effect on the response, the Elear is already at an advantage.
Second, the sensitivity of the Elear is also markedly higher, and so is the impedance. That means the output impedance of speaker amps' headphone outputs will affect it less, on top of which, you would need less power from them and therefore be farther from the point where they'll pile on noise and distortion. Normally a cheaper headphone would typically go for high sensitivity at the cost of other specs, while going up the ladder typically will have some engineers prioritizing a smoother response curve over sensitivity/efficiency, but in the case of the Elear, such a compromise wasn't made.
