While neutral means zero change in sound, I assumed by "a hint of warmth" you meant warmth added by the amplifier (which means it can't be neutral.) I apologize if I misunderstood your statements.
That the best advice. There's only one shop where I live (other than big-box stores which specialize in marketing-driven products I won't buy) which carries headphones. They don't carry headphone amps. Bummer for me.