I recently purchased a well-reviewed & very powerful headphone amplifier that I have running off my DAC, which outputs 3.0 volts single-ended (50% higher than the nominal 2.0 volts output standard that gave me no gain issues).
Well, between that hot DAC output & the lack of any gain stages on this new amp, I found myself in claasic too-much-gain territory--ie, using the amp's volume pot at the extreme counterclockwise rotation with almost no real-world volume adjustment possible before the volume on headphones became intolerably loud.
So I got a pair of the 10 dB in-line RCA attenuators by Rothwell. These work exactly as described, and are quite transparent sonically: I listened before and after installation (w/approximately matched volumes) and couldn't hear any difference.
So I recommend these attenuators.
Note that if this were for a huge, expensive 2-channel system in the living room, I might escalate to a pricey passive volume controller known to be uber-transparent.