Over the last few months I've tested 42 different Op Amps in order to find one that makes the XD-05+ sound just as good as a high quality desktop amp. In short, I failed. The chip that sounds closest to my desktop reference is probably about 90% there. 90% is certainly close, but I really was hoping for perfection. Having tried almost all of the reputable chips, I would bet the remaining 10% is probably some other part of the Amplifier that would need to be upgraded (capacitors, or some other part of the design).
Below is some info on the chips, all of which is entirely subjective and mainly applies to the XD-05+. This is likely also relevant to the regular XD-05 (for dual chips), but I've got no way to confirm that. My main focus was to find a sound that is faithful to the original intent, and as close as possible to what a desktop amp would provide. That said, some chips are just technically better than others even if they aren't truly neutral, so trying to rank them gets complicated.
The Winner:
AD827SQ : Tonally perfect, with a semi 3D sound stage and great details. With this chip the only thing holding the Amp back is a little more separation and refined clarity on every note. It's close, but a desktop amp will still sound better in a subtle but noticeable way.
Notable Runner Ups:
AD829SQ: Just like the AD827SQ, but a little thinner. May be preferable if your headphones are a little thick by default.
AD827AQ: Similar to the 827SQ, but without the 3D stage, softer details, and an extended treble. Costing less than half of the SQ, this is a great choice if you don't feel like spending for the SQ, and don't mind a treble that extends all the way without being sibilant.
AD829AQ: Just like the AD827SQ, but with a little less of everything (stage, details, separation, etc...)
Muses02: Slightly higher than neutral which leads to a much 'clearer' sound compared to the 827SQ. Very good separation and stage, but the downside of this chip is that it is brighter and thinner than it should be. May be a good choice if you have dark or thick sounding headphones.
Muses03: Brilliant stage and details, marred by a much lower than neutral tone. This probably has the best stage and separation of all the chips. The treble is very rolled off, leading to a much lower than expected tone. May be a great choice for those that have overly bright headphones, or those that are very treble sensitive. Those looking for neutral should look elsewhere though.
Burson V5i: The thread favorite. Going by majority consensus this is the chip to get. As far as technical performance goes, it is excellent. Great stage width, depth, and details. What keeps this from being my #1 or #2 is the much darker than neutral tone, and a subtly loose bass response. It's not as tonally low as the Muses03, but it's close. Every note is a bit off which is fine for most music, but with electronic music it's very noticable.
Other Chips worth mentioning:
OPA627: Renowned in many DIY forums for many different setups, but it just doesn't work that well in the XD-05. Produces a good sound, but overall flat and boring.
Muses01: Way north of neutral in tone. All notes are brighter than they should be. The only nice thing about this chip is how clear everything is, but that is just due to how thin the mids are.
Muses8920: Of the Muses this one is closest to neutral. Less bright than the Muses02, but just a hint north of neutral and a bit thin. It has better stage and separation than most regular chips, but lacks refinement compared to Muses02 and Muses03. May shine with the right headphone setup, and also costs significantly less than the rest of the Muses line.
OPA1612 (Stock): Similar to OPA627, this is also renowned in DIY forums. It seems like a technically capable chip, but something about it isn't right in the XD-05. Puts out a dark, muffled, and mostly flat sound in this application.
Here's the rest of the breakdown. This is
not ordered in a true ranking. I've got details on all the chips, but it would take too long to truly rank them. I doubt anyone cares which one is #37 as opposed to #38 (I certainly don't), so they are just categorized more in generalized groups:
| Op Amp | Single/Dual | Rank |
| AD827SQ | Dual | 1- Best |
| AD829SQ | Single | 2 - Excellent |
| AD829AQ | Single | 2 - Excellent |
| Muses02 | Dual | 2 - Excellent |
| Muses03 | Single | 2 - Excellent |
| AD827AQ | Dual | 2 - Excellent |
| AD822 | Dual | 2 - Excellent |
| AD828 | Dual | 2 - Excellent |
| Burson V5i | Dual | 2 - Excellent |
| AD811 | Single | 3 - Great |
| AD844AQ | Single | 3 - Great |
| LT1122 | Single | 3 - Great |
| Muses8920 | Dual | 3 - Great |
| NJM2114 | Dual | 3 - Great |
| OP275 | Dual | 3 - Great |
| OPA2134 | Dual | 3 - Great |
| OPA2227 | Dual | 3 - Great |
| OPA2228 | Dual | 3 - Great |
| OPA2277 | Dual | 3 - Great |
| RC4558 | Dual | 3 - Great |
| AD797 | Single | 3 - Great |
| AD711 | Single | 4 - Good |
| Muses01 | Dual | 4 - Good |
| OPA604 | Single | 4 - Good |
| OPA627 | Single | 4 - Good |
| AD712 | Dual | 5 - Mediocre |
| LF353 | Dual | 5 - Mediocre |
| LM4562 | Dual | 5 - Mediocre |
| LM833 | Dual | 5 - Mediocre |
| LME49720 | Dual | 5 - Mediocre |
| NE5532 | Dual | 5 - Mediocre |
| NJM4560 | Dual | 5 - Mediocre |
| OPA2132 | Dual | 5 - Mediocre |
| TL0171 | Single | 5 - Mediocre |
| TLO52 | Dual | 5 - Mediocre |
| OPA1612 (Stock) | Dual | 5 - Mediocre |
| AD823 | Dual | 6 - Bad |
| AD826 | Dual | 6 - Bad |
| NE5534 | Single | 6 - Bad |
| NJM2068 | Dual | 6 - Bad |
| NJM4558 | Dual | 6 - Bad |
| OPA134 | Single | 6 - Bad |
