The RE-ZERO doesn't need an amp.
I'm not sure what aspect of "underwhelming" you're worried about. The DBA-02 has aggressive highs but the RE-ZERO has fuller lows and with more kick. Both have forward sound signatures. The treble energy of the DBA-02 might be loved by some and off-putting to others. The RE-ZERO is more natural in tone and offers a fuller, more textured note. The DBA-02 has more speed and detail. The DBA-02 is more aggressive in nature. Both are arguably bass light earphones, the DBA-02 lean both in body of note and sensitivity on the low end and the RE-ZERO a little fuller in note but also rolling off on the low end. Both sound well balanced as both have a relatively linear frequency response, the DBA-02 comes across a little lean while the RE-ZERO is more natural. Back to back, I preferred the RE-ZERO. The choice between them really is a matter of personal preference though. We tend to like our sound certain ways, often the ways we are accustomed to be that right or wrong.
I want to toss out two other options for the price range we're talking. First is the Custom 3. This is a very well balanced earphone but with a thicker, more textured note than either the DBA-02 or RE-ZERO. It is aggressive in energy but not forward due to less dynamic compression than either the DBA-02 or RE-ZERO. It's more articulate than either too. The notes are thicker, less clean than the DBA-02, but BA earphones don't really get muddy like dynamics. Thick is often good since it adds texture and heft to lower notes which many BA earphones suffer from. They tend to retain a lot of speed and detail too despite being thick and, for a BA, sluggish. The Custom 3 does roll off on both ends kind of like the RE-ZERO although the low end does extend better and responds very well to EQing. The high end of the Custom 3 can't compete with the DBA-02. It just rolls off and disappears above 10kHz. It doesn't feel lacking, but it's not limitless up top like the DBA-02. The other option is the Triple.Fi 10. This is another thicker noted BA earphone. It's a little smoother than the Custom 3 but sounds more natural and refined. Dynamics are a little compressed and again makes for a slightly forward sound like the DBA-02 and RE-ZERO. It retains good articulation of note, great impact and presence, and the treble is energetic and sweet. The frequency response isn't the most balanced though and offers a more colored but fun sound. Bass is more robust than any of the others and treble has a more refined sense than the DBA-02. It does match the frequency response balance of the other 3 though, and with the emphasized treble, it can sound a little bright and lean in tone. The high highs and the low lows are more impressive than the other three. It does respond well too EQing.
Let's see, how about basic benefits of each, like why is each one special?
DBA-02
Good: outstanding note speed, extended top end, linear frequency response that's ruler flat above 500Hz, very revealing (quiet information brought forward, byproduct of dynamic compression)
Bad: compressed dynamic range (technically a bad thing but creates certain sound characteristics), treble is more aggressive/energetic than low end, low frequency notes are overly thin in body/lean sounding (even with EQing)
RE-ZERO
Good: natural sound that has a life-like sense, linear frequency response that's flat to 125Hz, well textured, natural impact and energy
Bad: roll off on the low end, compressed dynamic range (forward sound), not super detailed compared to BA earphones and some dynamics
Custom 3
Good: balanced response, thick note, very high dynamic range, excellent texture and articulation of note, very high level of detail (from good dynamics and texturing), highly revealing (again from the dynamic range, thick and well textured notes)
Bad: note too thick (warm/bassy sound signature), thick note veils subtle details, rolls off above 10kHz and does not respond well to EQing on the high end (BA driver incapable of quality information in the very high frequencies).
Triple.Fi 10
Good: excellent low frequency and high frequency extension, excellent energy and presence at both extremes, good note articulation, well sized sound stage, very refined sound signature, high frequency "sweetness" that is rare to find, outstanding low frequency impact and energy for a BA based earphone, one of the more mindlessly fun to listen to earphones I've used, life-like sound
Bad: uneven frequency response (colored sound signature), broad and moderate treble emphasis (bright tonality), very thick note creates a somewhat veiled sounding presentation, borderline muddy in the bass frequencies(odd for a BA).
Those are the kinds of things you're getting with each. In a sense, the Triple.Fi 10 looks the most wrong, however I personally feel it's the best of the bunch in terms of sound quality and performance. It is the most colored and the physical design does make fitment a mess for some. The Custom 3 is surprisingly good. The cable sucks, but the sound is extremely good, a half step down from the best of the best but also at a small fraction of the price. The high quality and capability and more even response will probably make this a better choice over the Triple.Fi 10 for most. The RE-ZERO, I like. It has a natural sense about the sound, a little forward and not as revealing as the others, but more natural overall than any of the others. The DBA-02 is impressively fast, detailed, and quite revealing. It is a lean sounding earphone though with a short note like many BA based earphones suffer from. Depending on how robust and bassy you like your music, this may or may not be a problem. You may find the speed and detail worth the trade-off, but I personally find a lack of dynamic range and lack of note thickness and subsequently texture a problem when trying to reproduce sound. Things suffer when you reduce these. Compressed dynamics create a more aggressive and forward sound but does bring forth the subtle details within the music which can be desirable. The reduction of note thickness can lose details within the music and as part of this makes the perception of sound stage, space, and presence suffer. Sounds don't really flesh out and realism suffers. On the plus side, the short decay makes for a very clean and tidy note which helps keep complex information clean and separated.
There are many trade-offs to sound reproduction. You sort of have to understand what you prefer and seek. Blindly, I'll point you to the Custom 3. Bang for the buck there isn't a more capable option. However, they've been discontinued for year and are harder to find and find cheap ($100 and less used). The cord is stupid but is super light and with the over ear design not microphonic. The Custom 3 is also a good candidate for custom molds down the road to which you can improve the sound quality more and end up with a better cord. I personally feel the Triple.Fi 10 is the best of the bunch in terms of both holistic sound quality and fun factor, but initial fitment can be a hassle. The Triple.Fi 10 is an odd earphone. You really don't understand how good it is alone. It's an earphone that shows itself only when you compare it against other earphones and finally realize what it offers. By itself, it's colored and meh. It's good and fun to listen to, but you can't recognize what it offers you over everything else. Compare it against 10-20 of the best universal earphones out there, and it begins to show you how good it really is. Again, this is another candidate for custom molds down the road if one desires to make fitment easier. For the $100 price point, I think the RE-ZERO holds a special place. It has a very natural sense of sound that is not all that common and does a lot of things right and very little wrong. It's not exactly godly, but it's not exactly expensive either. The DBA-02, I personally didn't like. I view it as a more specialty product suited for those seeking what it offers. The balance is great, the speed is outstanding, and it is a very revealing earphone, but I feel a lack of dynamic range and lack of note thickness and texture are big issues. The DBA-02 starts to sound lean and weak with the reduced heft and presence on the low end and a little aggressive and forward due to the smaller dynamic range. The revealing nature of this reduced range can also come across noisy/busy at times where a lot of the smaller details don't remain as hidden and subtle as they should be. The speed and high frequency detail will impress most though. I just think the overall presentation lacks more than any of these other earphones. Still personal preference wins all, so know what you want, know what you prefer, and pick the product that fits that best.
