I owned three different headphones,i.e. Senn HD-800; Audeze LCD-X and HiFiman HE-500 and they all have different impedances. HD-800 has 300 ohms, LCD-X has 20 ohms while HE-500 has 38 ohms.
Though it was claimed by the maker that EF-5 can handle headphones with impedance from 2 ohms to 2K ohms I found that EF-5 pushes low impedance headphones better than high impedance headphone. In order to make EF-5 matches better with high impedance headphone today I have increased EF-5's impedance by inserting in series a 33 ohm resistor to the headphone socket as shown in the following picture. Now the EF-5 pushes my HD-800 like a charm.
With the additional resistor being inserted I found that there is no loss in SQ when low impedance headphones are played. The only drawback I can observe is that the volume needs to increase by a bit to maintain the previous level of output when using low impedance headphones but on high impedance headpnones the volume output seems to be the same. As a matter of interest after this minor mod the volume setting are almost the same for my 3 CANs. When I listen my CDs the volume knob seldom goes over to 10-11 o'clock position and the sound level is more than sufficient to enjoy my music.
In the weekends I tried to find out the sonic performance of my 3 headphones I owned when being driven by EF-5.
My finding based on my personal preference is that EF-5 can drive HD-800 and HE-500 very well. The Bass and HF are all well presented and with good mid range. (EF-5 is powerful enough to drive these 2 headphones and the volume knob setting at 11 o'clock is more than enough for music listening.) Overall HD-800 is slightly brighter and I can hear lots of minute HF musical instrument playback (this is due to the headphone's sonic signature) while HE-500 gives more balanced presentation but I found it has slightly less clarity and musical separation than HD-800.
However EF-5 and LCD-X is not to my personal taste as the music is too dark and the HF has rolled off too much even though the vocal and LF presentation is exceptionally good.
Now the EF-5 has been run for more than a week and I have further auditioned its performance with HD-800 and HE-500. Tube used is RCA Clear Top (later changed to Westinghouse 5963 and eventually to Telefunken ECC82), I am surprised that EF-5 matches wonderfully well with HE-500 and makes this CAN very neutral. The music is very pleasant to my ears though the sound staging is not as good as HD-800. The dynamics is good and the bass is reasonably well presented. I can only say it is a very good partner.
As to the matching of EF-5 with HD-800 I found that overall the HF is too much extended and is not to my taste unless someone likes to hear all the very minute HFs. The bass and mid range is a bit lean but it does have good dynamics and impulse. It is still a good match but when compared with HE-500 I still prefer the HE-500.
Lastly same as my previous findings EF-5 is not a very good match with LCD-X as the HF is rolled off by a big margin though the mid range and bass is exceptionally good.
Edited by inter voice - 9/9/15 at 7:49am