The SR-40 is an electric model and needs no bias voltage. Since the one you sawing comes with the SRD-4, all you would need is a power amp/integrated amp/receiver and then you would hook up the SRD-4 to speaker outputs. The SR-40 would too work with any of the Stax amps. I have SR-80 and they have a very nice midrange, but are missing in bass and treble compared to best gaming headsets. I wouldn't pay more than $75 for them, and only if in excellent condition.