Input
The first thing that needs to be considered when using a pair of headphones for your Spectra V3 is the size of the output. Most metal detector headphones have 1/4" outputs, which is what the Spectra V3 requires. However, most headphones found in electronics stores (sold for listening to music) are 1/8" wide. This means that you shouldn't use your iPhone earbuds for metal detecting. While it's possible to use an adapter, it weakens the connection, so it's best to stick with a pair of headphones with a 1/4" output.
Stereo vs. Mono
The second thing that needs to be considered when choosing a pair of headphones for your Spectra V3 is whether or not they're stereo. The Spectra V3 requires that you use stereo headphones. If you use mono headphones, you may find that you're only hearing the beeps out of one ear -- or none at all. This is because mono headphones are only capable of receiving signals from a single channel, but the Spectra V3 emits two channels to the headphones -- one for each ear.
Volume Control
If you're not going to use Kellyco headphones made specifically for the Spectra V3, you should also make sure the headphones have volume control on them. Depending on the size and depth of a buried object, the signal that the Spectra V3 metal detector sends to the headphones may be weaker or stronger. If you don't have volume control on your headphones, then you may be missing out on weak signals, due to the fact that you can't increase the volume at all.
Manufacturer Made
While you can use a variety of headphones with the Spectra V3, there's an advantage to buying the Kellyco headphones that are specifically designed for this metal detector. The main advantage is that no research is required on your part to find all of the qualifications needed for optimal metal detecting. The wireless headphones made by the manufacturer are guaranteed to be compatible (unless the headphones are defective), and will give you all of the advantages needed to get the most out of your metal detecting experience. You don't need to purchase them, but if you're inexperienced at buying headphones, purchasing the ones made by Kellyco may be your safest best.