actually, the higher the "ohms", the more power is required to produce sound. So if the power to the detector audio is limited, having a lower impedance would technically produce more volume. To get the most volume out of a set of detector headphones, the impedance of the headphones should theoretically "match" the impedance of the detector. It is my understanding that the impedance of the X-Terra is 230 ohms. Although the UR-30's are considered 100 ohm impedance headphones, since Minelab wires the speakers in series instead of parallel (like many other detector manufacturers) 100 ohm UR-30 headphones will "become" 200 ohm headphones on the X-Terra. So if you want a pair of UR-30's, go for it.
I've used just about every major make and model of headphones with my X-Terras. Most of them reproduce sound effectively. But there are also other factors I use in determining which headphones I use. I want a set of headphones that block out most of the outside noise. I want a set of headphones with their own adjustible volume control(s) I want a set of headphones with a 90 degree plug. I want a set of headphones that have ample cord, without having so much it dangles in my way when I am detecting. And I want a set of headphones that fit well and don't cause discomfort after a few hours in the field. I have a fairly large melon (cap size 7 5/8 - 7 3/4, depending on whether I need a haircut or not) There are several brands that sounded good with my X-Terras. But all things considered, I use Gray Ghost headphones by DetectorPro. I have a set of Gray Ghost Originals and a set of Gray Ghost NDT's. The Originals were purchased new about 5 years ago. Every couple years I replace the earcup pads, just to keep them soft and comfy. (cost = $15 - one of the reasons I like the DetectorPro headphones) A couple months ago, I noticed a static noise developing in the plug portion of the cord. Not wanting to be without headphones, I ordered a new set of NDT''s. NDT stands for No Down Time due to the replaceable cord (and spare) that comes with them. When I received the NDT's, I sent in the Originals and got them back within a week. No charge! Another reason I continue to use DetectorPro products!!! They work well, fit nicely and are backed by excellent service.
If you asked 6 different people, you are likely to get 6 different answers. My advice would be to visit a shop with mulitple makes and models, and get one that fits well and sounds good to you. JMHO HH Randy
X-Terra 70, Garrett Pro Pointer, Wilcox Pro 100 Digger