Of those two, I’d probably go with the k7. The t50rp scales well with more power, so you could always use the k7 as a DAC preamp out to something like the a30 pro.
Love my Zen dac, it will run my t60rp argons to deafening levels. But it doesnt drive them well. Soundstage, dynamics, and bass suffer. The Senn’s fair a little bit better, but still like a little more on tap then the zen dac has.
K7 pro has a healthy chunk more power on tap. It would work much better with your headphones. It has a very neutral sound signature. Too neutral for my tastes.