mystique x se / class a headphone amp

Hi, I am thinking to get mojo mystique x se dac to compliment my ER4SR dead flat IEMs, I prefer a neutral response, it seems like x se is the best option available right now, yet I am confused to find the class a headphone amp to bring the best of x se and ER4SR, Pass Labs HPA-1 is the only option or anything else is better?