I did a slight journey during the last ten months amp-rolling my P5 in search of the "perfect" sound. I don't have a massive bunch of other equipment lying around to test against so this is basically my impressions using my ears, music and gear and not a real proper review but with a bit of luck it might help someone along the line. I'm not a pro on audio lingo either so bear with my feeble attempts to descriptions and enjoy the wall of text.
Source: Sony Z5compact (200GB memory card, Onkyo HF player) -> Mojo (Lossless or 24 bit tracks have been used in testing, mostly jazz and classical music).
Headphones: ZMF Ori Padauk, Oppo PM3, Sennheiser HD429
Cables: Forza Audioworks Android LOD, Claire interconnects and headphone cables (Claire Hybrid on the Oris).
Since I plan to use the P5 (trans)portable all tests are conducted using Powerex 230mAh 9.6V NiMH batteries, and by the way - they charge just fine in the P5 PS. In chronological order I tested:
OPA627 + Stacked BUF634 (Stock): Good tonality and pretty good soundstaging. What I really can't stand here is the aggressiveness of the 627. Way to "engaging" for my taste, but this is definitely one combo people will have differing opinions about. I can hear it's sonic merits, but it really isn't for me. I often felt exhausted after a listening session rather than relaxed.
NE5534 + transistor buffers (the second combo in the P5 box): Not bad at all, and certainly less aggressive then the OPA 627. I found the tonality and some other HiFi qualities of the OPA627 combo slightly better though (to far back in time for me to be more precise). In a choice between the two stock combos this however would have been my choice due to less in-your-face sound.
Muses02 + Stacked BUF634: I borrowed an unused Muses02 from a friend and Fiio uses this combo in the well-reviewed AM2 amp module so I had high hopes plugging this in. Very good tonality here from the muses02 and my favourite so far, but soundstaging was a let-down. Very wide left to right, but a loss of front to back depth compared to the other combos. This was mostly tested on the PM3s which are perhaps not soundstage kings either but it really sounded rather flat.
Burson V5i + Stacked BUF634: This combo I first tested on my PM3s and found it very enjoyable. Better soundstaging (more cube-like) then the Muses02 combo above however I had to turn the P5 to high gain and use plenty of volume to get good listening levels which put me off a bit (maybe the just 70db gain on the V5i plays a part?).I sat very happy with this for a while and then I bought a pair of ZMF Ori (The headphone formerly known as ZMF Omni) and now all I get is massive white noise from the V5i + BUF634 combo so back to storage with the V5i
The ZMF Oris needed the extra body from the P5 far less than the PM3s so here I actually considered selling it and just use the Mojo, but in the end I decided to see if I can improve the sound even further (was too late to send the P5 back to shop for a full refund anyway). Under all circumstances the P5 is slightly more powerful, especially with PS hooked up, and has a proper volume knob instead of silly plastic balls. So further down the head-fi rabbit hole we go:
Hiflight Topkit: Not really satisfied with the combos so far (on the Ori+Mojo combo the working options are no real improvements) I bought myself a Highflight Topkit and boy was I in for a treat. This really transformed the P5. Tonality has more or less always been fine on the P5 and this combo with the Oris was no exception - good from top to bottom (at least the way I like it... good solid body, nice midrange/vocals and polite but reasonable extended top end, good attack/decay of notes as well). The soundstage really opened up compared to the stock version, both in size and precision.
Muses02 + Topkit buffers and grounds: Now we're talking! Highflight mentioned in our conversations the buffers are the real stars of the Topkit and adding the Muses02 the improvement to soundstaging has indeed been kept and I feel the Muses02 has a slight edge in musicality compared to the Topkit L/R amps (I basically tap my foot and smile more but I can't really put my finger on it - this is ridiculously also how Muses marketing taglines from NJR goes but for once I actually agree on the buzz words). The difference is subtle though and I could have easily lived without the Muses02 if I had not tried this combo.
Since this was so utterly
I just had to try one more thing: Burson V5i vs Muses02:
Burson V5i + Topkit buffers and grounds: After previous experiences I was a bit cautious about the v5i and started with using my old HD429 as a sacrificial headphone and this sounded wonderful (no noise or anything), but moving on to the ZMF Oris I have horrible clicks, pops, high pitched noises, etc (not the white noise of the V5i+634 though). so this combo is a no go. I even think it smelled interestingly after a few seconds... Maybe it has to do with headphone impedance as well as buffer matching since the very easy to drive Senns worked fine? I have sold my PM3 so can’t retry with those phones, but at least Oris sends the V5i into "overload" for some reason.
As for the Burning question of V5i vs Muses02 I have no definitive answer. The V5i has the potential to be a killer OP amp with excellent sonic merits but it seems very picky on buffer and headphone combos so at least in the P5 my findings match those of ph0n6 - I would use it with serious caution... Using the HD429s and the Topkit buffers the V5i is more or less on par with Muses02, but since these are not the headphones I want to use I didn't test it extensively.
In the end I have bought my own Muses02 and use it with the Topkit buffers/grounds as a final solution. Compared to the bare Mojo there is a bit more body to the sound (this was far more needed on the PM3s then the Oris but still a change I appreciate) and I feel the soundstaging is a bit more natural, effortless and wider. All of the excellent prat and tonal qualities of the Mojo I adore are also preserved so this feels like a real win-win combo and the whole kit feels more like a decent desktop setup then a portable solution (a bit of a brick but no problem moving it around the house).