Ok so life happened and I didn't have time to do this until today. Went to a local shop that had lots of hi-end headphones and amps in store. I listened to the following:
Amps:
- Grado RA1 (solid-state, €800)
- Feliks Audio Espressivo MK2 (tube, €850)
- Manley Absolute Headphone Amplifier (tube, €5000)
- RME ADI-2 DAC (built-in solid-state headphone amp, €950, own unit)
Headphones:
- Beyerdynamic DT770 250 ohm (own pair, in bad condition)
- Meze 99 Classics, wood finish
- Meze 99 Classics, plastic finish
- Hifiman Ananda
- Hifiman Sundara
- Grado Hemps
- Meze Empyrian
- Audeze LCD-X
Albums listened to:
- Floating Points - Crush (tr: Anasickmodular, LesAlpx, Bias) - 96k uncompressed via Qobuz
- Kendrick Lamar - To Pimp A Butterfly (tr: These Walls, Complexion, Hood Politics) - 44.1k uncompressed via Qobuz
- Trentemøller - The Last Resort (tr: Evil Dub, The Very Last Resort, Into The Trees) - 44.1k uncompressed via Qobuz
- Future - EVOL (tr: Xanny Family, Ain't No Time) - 44.1k uncompressed via Qobuz
The goal of my testing today was to determine whether or not the amp in the RME ADI-2 was powerful enough to drive my DT770s with my preferred EQ curve. When listening at home, at loud levels, I started to hear distortion across the whole spectrum whenever sub-heavy 808 notes hit, despite there being about -3dB headroom left in the RME. My EQ curve has a bell (+6dB @ 50Hz, Q 0.5), a low shelf (+2dB @ 85Hz, Q 0.5), and a high shelf (-4.0dB @ 6.1kHz, Q 0.5) to smooth out the response on the Beyers.
I listen to mainly electronic music and hip-hop and generally lean towards bassy headphones, as my ears already accentuate the treble wayyyy to much. Sometimes I feel like I've been born with the RIAA curve already applied to my ears, so I need to compensate if I want to enjoy music

. I like to listen loud - not ear-piercingly loud - but the main point of using headphones instead of speakers to me is so that I can crank it, as I live in an apartment and have had enough of angry neigbours.
Anyway - enough talking - these are my findings:
- The headphone amp is indeed good enough- it is the headphone drivers themselves that can't handle the sustained bass notes at loud volumes.
- When testing on different amps, the same distortion issue happened with the DT770s. So you unfortunately can't transform just any pair of headphones into basshead cans, as most drivers will start crapping out with bass boosts.
- The ADI-2 amp is fine for driving 250 Ohm headphones at loud volumes, but only just that. When listening at the loudest level I liked, I approached about -3dB headroom on it.
- I preferred the solid-state Grado RA1 to the tube-based Espressivo MK2, every time, on every single pair of headphones.
- The Espressivo only exaggerated the highs more, and started rolling off some of the sub bass at around 30Hz. The soundstage was better on the tube amp though.
- For the listening experience in general, I would pick the Grado amp over the tube amp. As an audio engineer with a huge love for tube-based mic pres, this came as a huge surprise.
- The mids, soundstage and treble on the DT770s is much worse than I expectedwhen you compare them to other pairs.
- Mids are scooped and hard to discern, the soundstage is narrow (which albeit is to be expected of closed cans) and the treble is just harsh compared to every other pair I tried.
- I was planning to upgrade these with the DT1770s before going to the shop - now, I'm not so sure…
- The Meze 99 Classics are overrated compared to what you read online.
- The lower midrange is both tubby and imprecise - I had a hard time seperating everything that was happening in the 120-350Hz range, which was also boosted too much for my liking. A very mid-bassy headphone, not so much sub-bassy.
- Highs were recessed to my taste, but almost too much. Would not consider using these without an EQ.
- Soundstage is pretty much the same as the DT770s, but the DT770s have better bass and are more comfortable to wear for long periods - both pairs are very comfortable though!
- Both the 99 Classics and the DT770s need an EQ to sound nice - but after EQing, I preferred the Beyers.
- The Hifiman Sundara exhibited the same distortion at semi-high volumes with the bass boost in place, even on the different amps.
- For planar magnetic headphones priced a good deal above the Beyers, this truly came as a surprise to me, and cemented the fact that this was indeed a headphone driver issue, and not due to the RME amp.
- The Meze Empyreans are the best headphones I've ever listened to.
- After being so disappointed in the 99 Classics, I was NOT expecting this much of a difference in the Empyrians.
- Bass was plentiful, even without an EQ: neither tubby or overpowering while remaining extremely precise and voluminous.
- Mids. Oh my god, the mids. Incredibly three-dimensional. The seperation here was off the charts and there was no 150Hz boost like on the 99 Classics.
- Highs. Not in-your-face, but not recessed either. The dynamics were fantastic. On sharper recordings, they benefitted from a treble cut of about -2dB. Still, the best treble i heard from all the pairs. Not fatiguing at all.
- They sound good plugged into the RME, but they really came into their own with an amp. Obviously they sounded the best on the Manley, but the Grado amp held its own.
- For headphones that sound better than the LCD-Xs they're also lighter and more comfortable to wear on your head. At 430 grams you don't forget that you're wearing them, but I could definitely envision myself wearing these for long periods of time without neck strain, which I get from the Audezes.
- They handled the bass boost gracefully, even at high levels - but honestly, I thought it was too much to boost the lows on these. A slight treble cut is preferable.
- €3000
– but when you compare this to a set of hi-fi speakers and amp in the same price range, you're getting more for your money here.
- The Manley amp has no business being that much better than everything else.
- Until trying the insanely expensive Manley Absolute tube amp, nothing had blown me away yet. I didn't think the Grado or Feliks audio amps were a significant enough upgrade from the RME - yes, they sounded a slight bit better, but not the type of difference I would want to pay an additional €800 for.
- The Manley, however, is breathtaking. It made both the Empyreans, Hemps and Sundara come into their own - everything was better. Dynamics were more dynamic, bass was more defined and voluminous, and the highs were silky, detailed and much more three-dimensional than I have ever experienced digital audio before.
- At €5000, this is completely out of reach for me, and I need to have Will Smith come by and zap me with the memory eraser thing from Men In Black so I can enjoy my current setup again... If you can somehow afford this, this is a no-brainer purchase. But if you're making this kind of money to drop on headphones in the first place, you're probably not reading my low-end basshead thoughts, and are not considering the ADI-2 anyways

So, to sum up:
The amp in the RME ADI-2 DAC is absolutely fantastic. To approach its limits you need to have headphones that are at least twice as expensive as the DAC itself, and if you're approaching that price range on head-fi gear, you should definitely consider also getting an amp - but don't cheap out on one! If you want a noticeable improvement in sound, you need to consider amps priced above €1000.