Last I chatted with Yulong, he told me the A28 was actually like an evolution of the A100 amp. While A18 is warm and musical, A28 will be more neutral, like the A100 was. Not many people seem to have heard the A100 - it's a severely underexposed amp considering the level of technical ability it achieves. Maybe it came along too early before Yulong started blowing up.
Yulong still thinks A18 is ultimately the "better" amp - pure discrete output, more complex power supply, etc - which is reflected in the price. Seems like the D18/A18 are his musical line and the DA8/A28 will be his true balanced, more neutral configuration. If he can make the A28 as technically brilliant as the A100, but with the "soul" of the DA8.... it will be a winner for sure.