$20 CozyPhones flat stereo headband speakers sound terrific and let me hear on both sides with my Rondos in true separate left/right stereo. They are my go to option for my Rondos when I can not use the wonderfully easy Artone...which can be worn under a shirt and is tiny and light. So for me with my Rondos.... Artone is first choice, CozyPhones is second choice (planes or need to plug into a headphone jack) and mini battery pack with red jack DAI cable is third choice.
Things sound awesome with the mini battery pack and DAI red jack cable. It just is more cumbersome.
I like listening bilaterally always. (Except if I am training speech or speech in noise.)
There is a bilateral red jack DAI cable which can plug into the left and right mini battery packs or plug into the left and right Sonnet FM battery cover with the Euro jack (3 holes). There is also a unilateral red jack DAI cable.