It is a mistake to correlate price with quality, you'd be more correct to correlate the age (in terms of how old the design is) with quality. Today's best $150 amps/DACs far outperform the best of the earlier 2010s and 2000s, which is when many of today's most popular amps and DACs were designed. For example, both the $3,500 Benchmark HPA4 and $290 SMSL SH-9 use a pair of THX-AAA 888 amplifiers... yup, the same amplifiers! Along with the same type of volume control. Both generally use common, high quality components as well. They measure identically in every area except for power output (and probably crosstalk but this is a nonissue in pretty much any 21st century amp), no doubt influenced by their different power supplies. Gain is also something that might vary between them. On the other hand, there are plenty of technically poor performing amplifiers with 4 figure price tags (or even 5 figures in the speaker world).
For electronics (DACs, amps), a complete set of measurements really tells their story quite well, and level matched blind testing validates this, which is what has led me to part ways with all my overpriced audiophile brand electronics. These devices have a simple job. Power constraints? You can calculate the amount of power you need for a desired SPL for a given headphone, so you don't have to try a headphone+amp combo to figure this out. The E3 doesn't need that much power. Amplifier frequency response? Anything worth a damn will be flat beyond the audible band, including good $100 amps. Then you know the amp isn't affecting your headphone's frequency response. Noise and distortion (THD and IMD), SNR and SINAD? These metrics are dominated by affordable amps (along with that Benchmark amp).
That's getting off topic though. The E3 as said earlier is pretty easy to run: it needs an amp, a transparent amp is always ideal, but it doesn't need that much power. My Chord Mojo 2 handles it just fine. I haven't blind tested it against my Bricasti M1 SE or SMSL SU-X + HeadAmp GS-X Mini or Topping A70 Pro since it's annoying to set that up, but it certainly doesn't sound like I'm missing anything.
If you want to experiment with how the E3 sounds, it takes EQ well due to how low its distortion levels are. After doing so, I ended up back where I started though. The E3's stock tuning is really perfect for me.
Indeed, except with regards to point 1 if you're just power starved and can't get ample volume. Thankfully these days we can get top performing headphone amps and DACs for under $150 though, and these amps have a hell of a lot of power these days like the latest Magni and Atom Amp 2. JDS Labs and THX changed the game in the 2010s.