Thanks. I'm enjoying the Burson V5i in this amp a lot.
Well, it's a little more complex than saying the V5i is warmer sounding or has a wider sound stage than stock. There's a lot more going on.
First off, I actually did not listen to the stock op-amp long enough to recall if it tended more towards warm or neutral. It definitely was not cold sounding, and it definitely was not overly warm and syrupy. So the stock op-amp is somewhere in the middle.
If anything, the difference in warmth was such that it didn't stick in my mind as to which one was warmer sounding.
But here is the great thing about the Burson, and why I believe the upgrade is worthwhile: compared to the stock op-amp, the Burson V5i brings to the table a very satisfying sense of "easy power."
What does that mean?
The Burson V5i imbues the sound with a sense of solidity and weight. Bass sounds firmer and more grounded. Everything sounds sure and unconfused.
For example, tympany strikes sound tauter, firmer, with more initial impact and dynamics. It sounds closer to the energy of a real tympany strike.
Dynamics are increased. There's more contrast between loud and soft, not just on a macro scale, but on a micro scale as well. So there's the potential for more dramatic expression in the music.
But interestingly, along with this power comes a sense of ease. The music doesn't sound like it's struggling to get out. It doesn't sound impeded or held back. Nor veiled.
It has an easy, powerful flow to it. In that way, it sounds more compelling, and more realistic. More natural, too, since sounds in real life don't seem artificially impeded, or held back.
Another word for it might be unrestrained. Music through the Burson V5i sounds unrestrained. Powerful, sure, grounded, yet easy and unrestrained.
I think that is the greatest thing the Burson V5i brings to the music. And IMO, it's very worth the cost of the upgrade.
As for sound stage width, again I don't recall which one was wider sounding. But I can say this: with the Burson 5Vi, there's more of a sense of open space between instruments and voices. So I while I wouldn't necessarily say the Burson sounds wider (it might), it does sound notably more open and spacious.
Regarding the MUSES02, yes, it is a great op-amp and one of my all time favorites.
But when I tried it in the XD-05, the results were surprisingly "meh." Nothing terribly special about it, and maybe a tad boring.
Now the MUSES02 is a very rich, organic sounding op-amp. It really brings out the tones and textures of sounds, and excels in communicating the music with great emotion.
In the XD-05, it didn't really pull this off that well. While it did sound warm and emotive, there was something veiled and, well, restrained sounding about it.
I think the culprit here is the combination of the MUSES02 and the built-in BUF634 buffers. These buffers, themselves, err on the side of sounding a bit warm and opaque, just slightly thick. So combining the two pushes it too far into the warm and wooley camp, IMO.
I found this out by playing with the FiiO E12DIY a few years back. This amp allows one to change out both the op-amps AND the buffers, so a user can experiment with a ton of different combinations.
Here, I found that the MUSES02 work much better with a leaner sounding buffer. And in fact, the buffer that sounded best to me was the LME49600, which is a leaner, slightly wiry sounding buffer.
If fact, this very combination, the MUSES02 op-amp and LME49600 buffer, found its way into the FiiO E12A amplifier, which is a very nice sounding amp, IMO.
So I don't personally think the MUSES02 sounds the best in this amp since you are stuck with the BUF634 buffers.
As for the MUSES01, after trying the MUSES02 and not liking it here, I didn't even think about trying the MUSES01. I also believe the MUSES01 has some voltage or current requirements that don't necessarily make them a direct drop in replacement, not sure.
I have tried the MUSES01 in a JDS Labs CMoyBB amp, and it sounds glorious. But that amp had two 9v batteries in it in order to support the difference in voltage or current.
You should try rolling all sorts of op-amps to see what you like. For me the endgame op-amp in the XD-05 turns out to be the Burson V5i.
I hope this helps!
P.S. - I'm finding that this sense of "easy power" may be the hallmark sound of the Burson op-amps.
For example, I have a full compliment of the full-size single and dual V5 op-amps in my Gustard H10 amp, and the change was pretty much exactly in line with what I described above.