Another thing, people talking about Hugo 2 not having enough “drive”.
Objectively, it has way more “drive” than the vast majority of amps near its price point. It’s not a subjective analysis, it’s purely a measurement of output power against load. Hugo 2 can output a very high amount of current, with an extremely low output impedance - it powers the vast majority of headphones (only a minority may need extra power like the Susvara) to ear bleeding levels.
I use the Focal Spirit Classic, which is an incredibly underrated pair of cans - being one the most neutral pair of closed-backs on the market. With Hugo 2, everything one could possibly want is there. Heavy but tight bass slam, organically fluid vocals, airy highs, excellent depth and separation. I rarely go above volume red, not even with Audeze LCD-2 which I have tried with my Hugo 2 - while not as neutral as my Focals, Audeze bass is the deepest on the market, with vanishingly low distortion and perfect slam. I am never even close to maximum volume - which only clips in the white zone, something anyone who wishes to preserve their hearing will never come close to.
So why does using an external amp with certain headphones and certain individuals bias of what good sound is, appear to give their cans more “drive”? The main reason is very simple as Rob has explained; damping factor.
The Hugo 2 has an extremely low output impedance of around 0.025ohms. Which means it can accurately drive cans with impedance ranging from single digits to hundreds of ohms without changing the frequency response of the drivers significantly. The higher the output impedance, the worse the damping factor - which for most cans can result in a bass bump, here’s an example with HD650s:
(Scroll down the links page to the damping factor part)
https://diyaudioheaven.wordpress.com/headphones/measurements/brands-s-se/hd650/
As you can see, higher damping factor results in a bump of mid-bass. As Hugo 2 output impedance is so low, bass from every headphone will be incredibly linear - which may sound great with cans that produce excellent bass, but (subjectively) bad with cans that don’t have the best bass. The proper solution for bad cans would be to buy better/different cans rather than shelling out extra money on yet another box for your system - fix the problem at its source, don’t bandage the wound with an amp.
Sennheiser make the HDV820 which is designed with the intention to add extra warmth and bass to the HD800. Its poor damping factor is successful in giving the HD800 a bit more bass and tilting the overall balance away from its crazy treble peak - it also increases low frequency THD. If the HD800s didn’t have these problems in the first place, they wouldn’t need to tune it in such a way - do you get my drift? If you’re going to consider spend thousands on an amp to go in between the Hugo 2 and your high-end cans, first consider changing your cans. There’s a huge amount of choice on the market and it’s perfectly possible that you have picked the wrong set for your tastes. While there’s no perfect pair of portable transducers - there are some that are much closer to perfect than others.
Another thing of note, sometimes imaging (soundstage) can change when using an amp after the DAC. This is down to analogue bandwidth. Hugo 2 has an extremely wide bandwidth and incredibly phase accuracy - which account for its imaging acuity and depth. Any amp after the Hugo 2 signal will massively reduce bandwidth and phase accuracy, destroying all the original imaging nuance and harmonic separation - in favour of “bigger”, rounded off imaging. This is exactly what the Hugo 2 does when you switch it to the Mojo filter in a way.
