Not an expert but your headphones are probably enough load to keep your transformers from being damaged but I'm not sure. The Senns are listed a 300 ohm which is kindo high. The Hifiman are 50 ohm which should keep your amp happy.
I'd be trying to figure out why my system is so harsh.
What are you feeding it with? Many DACs and preAmps are also a headphone amp which would make more sense. Your amp is heating up the room with 300+ watts of power to drive a load that a preamp can drive.
Jerry