I received a pair of Burson V5i single op-amps a few weeks ago and I must say that these little silvery guys are incredibly great sounding in this amp.
I've let the op-amps run in the Millett Nu-Tube amp for about a 100 hours total, just to let things settle in, as things tend to do...
The op-amps that PM provide on the BOM list, are actually good sounding already. They sound clear and concise. I wanted to see if I could find a more tailored sound to my liking though.
The Burson V5i Singles sound different from those on the BOM list. I think they sound much better. They have a certain enveloping characteristic that is very engaging and lush.
I think that they add depth to the soundstage and have a very pleasing and clear vocal presence. I especially enjoy how these make a female voice sound bold and present, not tinny or thin. Just great here.
The V5i also adds a nice pillow of bottom end that makes the Bass sound closer to what I like to hear...energetic, bouncy, and well rounded. It's just does a great job of enveloping you with sounds instead of presenting them in front of you. With the V5i, I think I have found my personal favorite so far in an op amp for this amp.
Now, I also have the Sparkos Labs singles. I like those too, but I actually think, in this case, the Bursons beat those ones out. The Sparkos Labs singles are extremely up-front and bold. It's like everything is turned up to 11 on those (I even turned down the gain to about 10). Bass is up, Mids are up, Treble is up. Those are very clear sounding too, but are a bit fatiguing to listen to for long periods of time. The Bursons just have more space and lushness with it's clarity.
I also used a Pono Player, Shanling M3s, Aune M1, and a Zishan Z2 as my sources. You know, source matters. I found that the Pono Player had a nice mix of analog quality and clarity through this amp. The M1 has a really bold LO, but very clear sounding. The Shanling was decent but not very warm. The Zishan Z2 just delivered a full analogy sound which is pretty impressive and headphone filling.
Also, I used Sony Z7, Sennheiser 650, Beyerdynamic T90, Massdrop AKG KXXXXX as my headphones. The Sony's are the best match here for me with it's big soundstage and bolder bass. I also really liked the AKG set with this amp and the Bursons, a really spacious musical pairing.