GSX mini vs Burson Soloist 3X
So I’ve been listening to both of these amplifiers to see what the fuss was about. And I think I get it now. Well, sort of.
Keeping in mind:
The GSX mini is an $1800 fully Class A amplifier with an optional $200 DACT attenuator upgrade.
The Burson Soloist 3X is a $1145 fully Class A amplifier with optional op amp rolling options.
I did all my listening using headphones via the balanced 4 pin XLR outs on both amplifiers.
I don’t think you should use these amplifiers single-ended unless you have to.
As far as power, GSX mini seems to have more power, especially at higher impedances.
Mini: 6W at 25 ohms, 4W at 50 ohms, 2W at 100 ohms (from HeadAmp’s site)
Soloist: 8W at 16 ohms, 4W at 32 ohms, 1W at 100 ohms (from Bloom audio)
First the GSX mini. The music sounds powerful yet smooth and controlled. The impressive technical performance is really apparent. The background is shockingly black with no music playing.
I think the amplifier is mostly very neutral sounding though it might slightly lean bright. I have not heard the GSX MK2 but some say that the Mini retains some of the signature brightness from the MK2. The amplifier is also quite holographic and full bodied. I really like it. When I first had this amplifier I compared it with my Topping A90 and the A90 sounded like a brick wall of sound in comparison to the holographic GSX mini. It’s probably not tube amp levels of depth or staging at this price but it’s still impressive. Not everyone wants to deal with tubes or distortion.
Now the Soloist 3X. With the default red Vivid op amps installed I liked the presentation the most. It wasn’t trying to outright impress me in its technical performance but it was more fun and laid back and surprised me occasionally with its dynamics. It didn’t have as much weight to music but was more airy sounding. It also retains that holographic presentation to my ears, which I believe people associate with Class A amplifiers. It’s not as bright as the mini, but I suppose that means it’s closer to neutral? It’s certainly not warm though I’m sure my Bottlehead Crack has skewed my perception of what warm is.
I wonder where this stereotype that “Class A amplifiers sound warm” came from. 
Anyways when I swapped to the orange Classic op amps, I could notice a slight edge to the music now. I wasn’t particularly fond of it to be honest. If you prefer that sound then more power to you, but I think it made the amplifier too lean sounding for a marginal detail improvement. The op amps also cost a pretty penny.
As a bonus, I swapped in some Sparkos Labs SS3602 dual op amps. Unlike the Burson op amps, you need to be extra careful to orient them properly or you’ll probably damage the amplifier. The Sparkos were quite different. They sounded very clean, wide, and detailed. Unfortunately this came at the cost of being too bright. Brighter than I was comfortable with as my planar magnetic headphones especially just sounded sibliant.
To my ears the Mini has better resolution, imaging, and fullness to the music overall. Partially due to more power I’m sure. HOWEVER, as everyone knows you really need to get into synergy balancing. The DAC I used throughout this comparison was the Schiit Bifrost 2 which some have called warm.
If the Bifrost is masking the GSX mini to seem less bright than it is, that should be accounted for. If you have a brighter, harsher sounding DAC you may prefer the tonality of the default Soloist over the GSX mini. As a rough estimate, I think the Soloist hit around 75-80% the technical performance and fullness on the GSX mini for my planars and 85-90% the performance on my dynamics.
I prefer the GSX mini more on my chain and I hope to upgrade my DAC and tube amp in the near future. (Holo Spring 3?)
TLDR
To my ears, you get what you pay for but make sure to balance the synergy in your audio chain.
The Mini was fuller bodied, brighter, more technically capable, and more powerful.
The Soloist was more fun, neutral, airy, and has the added benefit of op amp rolling.
As far as my preference:
For my planars the GSX mini was a clear cut above the Soloist.
For my dynamics the GSX mini was still the winner, although by a slimmer margin.