I'd vote for any or all of these products:
Topping A30 headphone amp
JDS Ol switcher passive preamp
JDS labs Subjective3, 3 band equalizer (very new, very interesting)
magni 3
O2 amp (have one with 1x/3.3x gain, pm me if you need it for review)
Aune X1s 10th anniversary dac/amp
Arcam rHead amp...