I wanted to use the Monitor1 volume control as a safety between my Audient ID44 interface and IK MTM monitors, using Mogami 2534 cables. Its simplicity was quite promising and I tried it for almost a month until I finally gave up.
The good: It's a well built, pleasant looking device with a good feel to the enclosure, buttons and the volume knob. It does not come with any bells and whistles and makes a good down-to-earth impression.
The bad: Passive attenuators should be used at or near full volume, otherwise they will impact the sound signature. However, this one is quite disappointing in the entire volume range.
First, you lose quite a lot of sound resolution with this device. It has poor channel separation and the soundstage becomes small and flat, particularly at low volumes.
Second, as is common for passive attenuators, the tonal balance changes with the position of the volume knob. However, here I found no position that would sound good: as you turn the volume up, you get from quite dark to bright-ish but with an unpleasant sharpness in the highs. It can sound fine with some recordings, but the minute you bypass it in your audio chain, you will see how much it has taken away.
Ultimately it was replaced with an SPL Volume2 and although by no means transparent, it preserves most of the imaging and its colouring, although noticeable, is much less invasive.