So, it seems that I don't have any headphones that this Ampersand cannot drive well, with more than enough power for even the Original HiFiMan HE-6. However, as a "wire with gain" it sounds just like whatever source I use. So, if you pick the wrong source then the ampersand can sound as bright or as dark as the source. I find it to be utterly transparent, and a viable replacement for most desktop headphone amps as well.
My PALAB M1 Mini is not a good choice as the source when listening to HD-800/800s, as it is a little too bright for those; but wow does the M1 Mini + Ampersand sound good with my darker sounding HD-600 and HD-6XX!! Even the slightly brighter Grado HF-2 LE sound good with the M1 Mini alone or via Ampersand. But not so with the brighter HD-800/800s.
I've reported in the past that the Palab M1 Mini can drive the HD-6XX with authority by itself with the balanced output, while in that review I had found that the HD-600 sounded better with the HiFiM8 V2. But, now I found the HD600 were also very good with the M1 mini if I feed the M1 Mini into the Ampersand first and then plug the HD-600 into that.
My BlueDAC is a warmer sounding amp with slightly more bass, but it's great as the source/DAC for feeding the Ampersand with just about any headphone. I prefer this BlueDAC and my HiFiM8 V2 as the source, because the combo is more versatile and I can listen with anything via the Ampersand. I think the Ampersand is a bit much power for the HE-1000se and Edition-X V2, and I turn the gain down to medium for the 1000se and low for the Edition-X.
The BlueDAC can drive the above on its own without the Ampersand, but when it comes to HD-800/800s or HE-6 then YOU REALLY NEED the AMPERSAND. The HD-600/6XX sound good via blueDAC balanced 2.5mm output, but even better and more punchy when feeding the BlueDAC into the Ampersand first.
The HiFiMan R2R2000 is also a good source with the HD-800/800s and is necessary to properly drive the HD800/800s when you have the R2R2000. But as I said, the HE-1000se do not need a stronger amp than the R2R2000, HiFiM8 V2, or BlueDAC. Still, the HD-800/800s sound much better from the Ampersand than directly from the R2R2000 built-in amp (which is fantastic without an external amp for my easier to drive headphones like HE-1000se, Edition-X V2, or IEM like Westone W80 and Westone ES60, etc).
The HiFiM8 V2 can easily drive the HE-560 V1 balanced, but the Ampersand gives them more punch and authority as well, like with the HE-6 V1. Again, the Palab M1 Mini is not a good source/DAC for the HE-560 V1 as it makes them a little bright, while the BlueDAC or HiFiM8 V2 + ampersand pair with them much better.
I have not tried easy to drive full size headphones like CEntrance Cerene dB or V-MODA M-100, as it would be pointless based on my experience with the Edition-X V2 which are easily overpowered by the Ampersand.
I am amazed.