No doubt you could find cheaper than that Sennheiser amp (although when compared to the price of your headphones it's arguably not that expensive)
I can see how you would want to make the best of the headphones you've got, but you may want to have a read of this, from a highly respected company, saying why they don't bother with balanced h/p outputs, even suggesting that the sound could be worse (copy / paste necessary for the link):
https://benchmarkmedia.com/blogs/application_notes/audio-myth-balanced-headphone-outputs-are-better
As you might imagine, there are others saying why balanced beats unbalanced with headphones.
Don't get confused if you read how balanced connections between professional audio equipment are an advantage - that's different (that's what the balanced input on the rhead is)
I'm a big Sennheiser fan (once had a listen to a £9,000 pair of theirs, complete with their own valve amp) and I've got their HD590 model, among others.
I reckon yours would probably be great with (just) the Mojo, using the standard lead and 3.5mm adapter.