Do you already have a decent DAC for this setup?
Below are my recommendations for the amp.
Amp1: Drop THX AAA 789
~$350
Has both single ended and balanced headphone jacks, tons of power, 3 gain settings, XLR and RCA inputs, auto off feature, and sounds and measures excellent
Amp2: Schiit Heresy
$99
This is a single ended only amp but measures and sounds awesome. RCA inputs, 1/4 headphone jack, cooling vents on top although it never gets passed warm, small footprint.
I own 2 of each of these amps. I have one for work and one for my hi-fi setup at home and cannot recommend them enough. Great value, awesome sound, near perfect measurements, and aesthetically pleasing.
