I have had in house the following:
SimAudio Moon 430HA (current)
HeadAmp GSX Mini (current)
Luxman P750u (1 month demo)
Violectric V281 (2 week demo)
Benchmark HPA4 (2 week demo)
dCS Bartok (owned for 2 months)
Headphones used are Focal Utopia and Sony Signature Series Z1R. I also have a 2 amp 2 headphone Stax system however for this exercise it was not applicable.
Source used for demo - Linn Klimax DS
In a nutshell the keeper here is the (relatively) new HeadAmp GSX Mini - which is also reputed by some to sound better than their flagship GSX MK2. At $1795/$1995 it is amazing VFM.
I have never liked the SimAudio HA and have just been too lazy to sell it.
The Luxman was an impeccably built piece of equipment however imho it was let down my its LEUCA volume control.
The V281 was a stellar performer - however I preferred my GSX Mini.
The Benchmark HPA4 was a stellar performer - however I preferred my GSX Mini
The Bartok is an amazing all in one solution however, for me, while dCS first attempt at a headphone amplifier is highly impressive, I preferred the performance of the Bartok as a DAC/Streamer only and I used the HeadAmp GSX Mini for headphone amplifier duties. The Bartok’s performance as a DAC/Streamer gave me enough incentive to try a Rossini/Master Clock and this has now replaced my Linn KDS in a Naim 500 system.
Hope this helps. As always do the demos - your mileage may vary.
Best
Gregg
PS a good friend who also runs a totl Naim system for the past 25 years acted similarly and returned his demo Benchmark HPA4 and has remained with his Violectric V281.