The ES9018 chip requires a good power supply for its 80MHz clock, it works best in current mode (using opamp amp as I/V would immediately cut at least 8db of SNR). It is also a 8 channel DAC chip so balanced output configuration would better utilize it.
IMO, the Minimax's basic power section, NE5532 opamp I/V stage, and RCA only output stage are suffocating the ES9018 chip. I can't help but wonder the tube is just there to cover up all the design weaknesses. The strengths of the ES9018 are its high dynamic and revealing power. The Minimax implementation is like lowering the suspensions, putting spoilers and low profile tyres to a Land Rover SUV.
If you order the NFB-1ES, ask Kingwa if he still has the 1ppm 80MHz TCXO as upgrade.
I'm not questioning your comment and I haven't heard any GD DACs with the ES9018 chip but I do know that despite its "flawed design" the Minimax does deliver the goods soundwise. I would really like to hear what a "well designed" 750$ DAC that doesn't suffocate the ES9018 chip sounds like.