Doug, I would not order a replacement from Kellyco unless you upgrade, the volume control on the earlier phones were susceptible to dirt, water, and wire fraying. I ended up removing my Explorer headphones volume control. I'm wondering if that's why yours doesn't have one, if Minelab did away with it.
The 1/8" inch phone jack is also going to be a problem, I'm assuming they didn't change that yet, so you may want to consider picking up the Sunray headphone jack adapter/stand. You can still use the phones you have by picking up an 1/8th" to 1/4" adapter, 2 bucks I think. The Explorer headphones aren't that bad, I used them for 5 months with excellent results, however they are not of a high quality and you can do better with several different brands to choose from.
Volume does not effect performance in terms of the actual depths and cut-ability, if the Explorer is seeing down 8"s at volume (5) then at volume (10) it is seeing down 8"s. Volume and Gain settings do more in the area of 'alerting' you when a signal is given, if it's a quick weak signal and your volume is set too low then you might miss it. By running at maximum volume you are insuring that you are doing all you can to hear that signal, and by using a better set of headphones that can achieve a higher volume then you are insuring even better that you wont miss a target. A hunter with good hearing may not need a great set of phones, but any difficulty in hearing will have a call for a higher volume and better phones.
I use higher quality phones now, and believe they have given me an advantage over the Stock phones, but I would not consider throwing the stock phones out because though a lower quality, they are still pretty good and an actually louder that some other phones I have.
GL
