It's not really a concern as any available headphones will work, from 8Ω to several hundred Ω. It is commonly believed though, that higher impedance requires less power to drive, and that they'll be more sensitive to faint signals.
I believe it was Allan at Tesoro who told me that the inexpensive Calrad 15-135b will work just fine and they do...but they don't sound good to me; sort of muffled. Pretty sure those Calrad are 8Ω. I've also used cheap music headphones which are commonly 32Ω. They sounded great and were comfortable, but are comparatively fragile and without volume control.
The Killer b's are 150Ω, BTW, and are made specifically for detecting. I was skeptical as to their value before buying mine, but I really do enjoy them.
Folks, they're really not that much $ and do have a lifetime warranty, just like your Tesoro. We're talking about roughly a hundred bucks for Kb's headphones here, and not $300+ for audiophile phones (which would still be fragile and not built for detecting).
I think it's funny that people don't want to spend another hundred on a good set of headphones that'll be on your skull for several hours a day.. I guess it's hard to get the $100 ones, when you see so many you can use, that only cost $30... I get it.
BTW - I was the same way, until I broke down and bought my Kb's.