resonance at 500 Hz using headphones - maybe need a different DAC?

I'm primarily a headphone listener (Audeze LCD 2 pre-fazor) and I've been troubled by a resonance around 500 Hz that seems curiously dependent on my digital source. I'm looking to troubleshoot this resonance.

Note that all my equipment has modifications by a technician in the N.J. Audio Society. He adds bypass caps, damping, and ERS fabric.

I was using a modified iFi Nano for a while and liked it (with linear 5V power supply). Then I purchased a Gustard x20pro DAC, had the guy tweak it out. I noticed at first that the sound was much improved compared to my iFi Nano, in terms of an immediacy, transient power, dynamics (micro- and macro-), and so forth, but I was getting something undesirable, which was a resonance in my headphones on certain notes.

I have a musical keyboard and piano synthesis software on my MacBook, so I played different piano notes along with a spectrum analyzer plugin and a parametric EQ, and determined that the main resonance was happening at approx 500 Hz. (C above middle C). There was another resonance an octave above that.

This resonance is particularly noticeable on choral music.

Furthermore, I've made several improvements to my digital sound, like adding a better power cable to the Gustard, experimenting with an Aurender N100 music server, and experimenting with an iFi Zen Stream with different power supplies.

Whenever I tried something that improved the USB source (like adding a better power supply on the Zen Steam, putting a better power cord on the Aurender, etc.) the sound from the Gustard became even more immediate, involving, dynamic etc. But the more of this I was getting from the Gustard, the worse the resonance became. 

I tried two other DACs I had lying around. I went back to the iFi Nano. I also tried a Beringer 24/96 DAC from a long time ago (it's modified, runs off battery supply and is actually pretty good). Both of these DACs had much less resonance or maybe none. But they were less dynamic, transient speed, involving, etc. than the Gustard. Very dynamically tame compared to the Gustard and that's a bit disappointing were I to go with these DACs long term.

So I may want a new DAC if this is the fault of the Gustard, But that's what I'm trying to determine... could a resonance be related to some general good quality of a DAC, like transient speed? Or is a fault in the Gustard?

Before I decide that I want a new DAC, I just want to debug this resonance. If anyone has encountered something like it or has a theory where it comes from, I'd like to know.


+1 @koestner First, this doesn’t even sound like a DAC problem, and second since it happens to some degree across several DACs it’s most likely not a DAC issue and more likely an issue with the headphones.  Have you even tried different ‘phones?  What are you using for an amp?

It only happens with the Gustard. Did you think the other equipment I mentioned like the Aurender or Zen Stream were DACs? Those were servers/streamers driving the USB input of the Gustard.

I've tried two different headphones, two different headphone amps, and when listening closely to speakers there seems to be some of the peakiness, too.

I did another experiment which helps to convince me that it's the Gustard. I set up my modified iFi iDSD Nano, which is using a very hq linear power supply, and I put my best power cord on the supply, using the Aurender as USB source. The sound had a lot of the qualities of the Gustard, in terms of transient speed, dynamics (micro and macro) etc. But almost no resonance. I then played piano notes through the DAC, and almost no resonance. 

What the Gustard is doing technically... frequency response, etc. ... I don't know, but somehow it makes this resonance worse.