Not sure what volume control they are utilizing, but I can provide my experience with getting the volume control on my Amber 3.
I almost bought a used Lampizator prior to the Amber, that had no VC...and I am so happy that I didnt end up getting that one. The overall sound quality of the Amber with the built in VC is stunning. To run a separate high quality preamp, that would offer the same SQ you would have to spend a lot of money (thousands $$$, I am sure.) Way more than adding the VC to the Amber costs.
On top of that, you dont have to buy another expensive power cable and interconnects. On my Amber, the interconnects coming from my streamer and the power cable supplying the Amber are another few grand. To replicate that same setup on a separate preamp would be costly.
I had a beautiful Conrad Johnson preamp, that I had purchased not long before the Amber. Part of justifying the Amber purchase for me included selling off that CJ to raise funds. I was sad to see it go at the time.
But now that I have the Amber, I have absolutely no regrets.