Creative Soundblaster X7 as DAC with 2x Sparkos SS3601 single op-amps, 2x Sparkos SS3602 dual op-amps, Hifiman HE400i, AKG K712 Pro, Fostex TH-x00 PH, Fidelio X2, Little Dot 1+ with Mullard M8100 tubes and of course the Burson V5i op-amp.
The LD1+ was already using a different op-amp, the dual op-amp that I swapped out of the Creative X7 was being used which already made for a clear upgrade from stock. The X7 op-amp upgrade seemed to give the sound a more neutral signature, while also lowering the noise floor with the LD1+.
Because of this, I wasn't expecting to have as noticable of an improvement with swapping in the Burson V5i, but I was happily mistaken.
The differences aren't as large as swapping all for out of my X7, but they were immediate. The LD1+ is not by any means an expensive tube-amp (well, maybe to those who don't use these forums!) However, the sound didn't improve in one area - it was improved all around, while retaining the neutrality I noticed with the X7 op-amp upgrade.
Details, transparency, cleanliness, soundstage, imaging, and a blacker background were all improvements I could notice on all of my headphones. Some more than others, such as the K712 Pro and HE400i seeing as these are harder to drive headphones I could only assume.
The Fidelio X2 still had some background fuzz when the volumes were at pub-style band performance levels but this is a reasonably low impedance, easy to drive headphone.
I'd also like to point out exactly what I mean by "cleanliness" because this was a very appreciated change.
I like to EQ, some are are against this, I am not. For example: for the X2 or TH-x00 I will EQ in a little bit of mids, and I will EQ some low-end into all of my headphones.
The LD1+ with the warm, buttery mullard M8100 tubes would become syrupy sounding while running high levels of bass (only as high as what the X7 as my amp would reproduce cleanly).
Now with the V5i op-amp, the bass was no longer thick and syrupy sounding, so creds for this.
Overall, this is an upgrade I could easily recommend if you are a owner of the Little Dot 1+ and want to go a little farther with your amp, without shelling out for a whole new tube amp.
I have not used the V5 to compare, but the V5i having a smaller footprint is a positive in this case as closing the LD1+ with the backplate remains possible.