Yeah, you could lower the impedance of the output tubes by adding some transformers. But then, it wouldn't be a Output Transformer Less design anymore. S.E.X. has transformers and drives about any headphone you can throw at. Now you have to live with the output tube impedance and choose carefully some really nice sounding, high impedance headphones : Beyerdynamics in the 250 to 600 Ohms (T1, 990, 880, 770), Senns : the HD650, 600 (300 Ohms) and of course the flagship HD800 (300 Ohms). Some say that the HD700 isn't the best choice at 150 Ohms. And some like the Crack's musicality even with lower impendance headphones.