As I remember Whites wants at least 60 ohms. Most of the second party phones run 150 ohms. (Gray ghosts, Sun Ray, etc.) I use Rat Phones on my MXT and they are 300 ohm.
I think phones should have volume control. I my case the MXT doesn"t have a volume control on the detector and it is loud. I run a limiter circuit on my phones. This allows me to turn up the phones to hear the quiet signals but limit the signal to a safe level on the shallow signals or large signals.
Some phones have a switch that allows them to work with most detectors. There are two ways manufactures wire the jack at the end of the cord. My rat phones have a cord that does this instead of a switch. Phones are rated as to how much outside noise they block. The more noise they block the easier it is to hear a signal.
Fit, comfort, and audio quality are a personal taste thing. Rob