Whether you use one extension or two is not critical in the situation you describe, as long as it reaches ear level in back.
I use a plug when using my KN7000 and position the Bose off to the side and slightly in back of me. This is the most common placement.
You should be fine. Good luck.
pa4X 76 ,Audya 76,Yamaha S970 , vArranger, Hammond SK1, Ketron SD40, Centerpoint Space Station