I believe the M40fs models mentioned here are 1/4" jack (no adapter on end), and Don is looking for 1/8".

FWIW I have no issue with a good extension cable, and use some that adapt the 1/4" down to 1/8" (solves problems sometimes).

Anywhere from 3' to 25' is used here depending on the need (sometimes even doubled for longer runs), and I don't recall ever having them come apart during use.
Even 9' seems like it would be restricting to me.
If you have a drummer. bassist, guitarist, keyboard player and vocalist all running off one headphone amp, you need good extension cables. That's why they exist.

I do not work here, but the benefits are still awesome
Make your sound your own!