Seaside - exactly. As we know, TPA6120 works fine with 2v/v gain (that's written in app note) - it's good for low imp. headphones. For higher imp. higher gain is needed. So that's why i used preamp before tpa. Another reason for this, is that TPA is some kind of "clean" headphone driver. It doesn't add anything from itself - so now, opamp can change sounding
OP275 is the best in my opinion.