I always find these questions and their replies slightly amusing. Will the X3 drive it to "their potential" - what is that?
I think we can assume he means X3ii as that is the thread he's posted in - if not, he can tell us.
I have the HD600 and the X3ii - I'm listening to them right now. So lets look at a couple of real world situations.
Dire Straits "Sultans of Swing" - 80/120 on low gain on the X3ii, no EQ or replay gain or anything else which can affect things. Measured with a calibrated SPL meter, the end of the mic just at my outer ear. Volume (A-weighted) is 65-75dB. Engaging high gain puts this into the 75-85 dB range. Same track on 120/120 puts it into 85-95dB range. 80/120 on low gain is pretty comfortable for my preferences. Above 100 with this track and its simply too loud for me.
Switch tracks to Amber Rubarth's "Tundra" - a binaurally recorded track, with a lot of dynamic range, and recorded quieter than a lot of my other classical music. Comfortable listening level for me is around 100/120 - so its getting near the limits of the X3ii, but still sufficient volume. At 120/120 on high gain, its putting out dynamic peaks at 85dB. Again for me - 100/120 is sufficient.
So a lot will depend on your preferred listening level.
So lets look at the theory of driving to "their potential". I also have (in front of me right now) L&P's flagship LP5 Gold DAP. I've had this at Meets before driving people's HD800s and had them comment that it sounds better than their full sized home rigs. It has the ability to get the HD600s (or the HD800s for that matter) well above 100dB, so power isn't an issue.
Both DAPs volume matched to my preferred 65-75dB range - playing the same track - and able to use fast switching so I can go from device to device at same point in song rapidly.
- Both sound absolutely excellent with the HD600
- Both have enough volume for my preferences (YMMV with this)
- The only minor differences is that the LP5 Gold's bass response is just a little warmer (fractionally) and the lower treble on the X3ii is the tiniest bit more prominent. The LP5 Gold appears just a little smoother. it is not "night and day". This could be the tonality differences of the amp sections. It could be that the LP5 Gold is slightly on the warm side, where X3ii is a little more neutral to cool.
If you're worried about power with the HD600, then an amp like the FiiO A5 will get you all the headroom you'd ever want, but even the E17K will give you a lot more headroom - and the tone controls on that amp are brilliant if you're trying to EQ a headphone slightly and you'd rather not worry about playing around with EQ curves/settings. But for me personally (at my listening levels) the X3ii has no issues actually driving them. I would probably tend to use the X3ii with the E17K though if I had a choice (more head-room, adds easy EQ options, and I like the volume wheel).
Hope this helps