When you use an "idiot light" device like a DD1, you have no idea what it's really doing. You have no idea if it's accurate... and from more than one review, they seem suspect. All you know is "I spent more money than a single channel o-scope" and "a light comes on at some point".
You have to TRUST that it comes on with 1% distortion, but without an O-scope you can't verify that. There's a very real chance that you use it to set up your system, and your system never reaches its full potential.
With an O-scope, you can actually see the waveform change. You can combine that with your ears to set your gains how you like. Maybe that's turning it back to 0% distortion. Or maybe it's 0% distortion on your tweeter amp, 3% distortion on your midbass amp, 10% distortion on your sub amp.
Even the pixellated screens of the OG Velleman like I still have - it's plenty good enough. You can change the range and scale, but auto-range makes it quite simple to use. ...and there's no more pixellation possible than a single pixel idiot light on a DD1, right? [emoji6]
Gain setting isn't idiot-light simple. The O-scope is the way to go.
Plus, in addition to what Geo said, you can even run frequency sweeps on your amp and see if it's actually flat at all frequencies. Same with any other components in your system.
An overpriced idiot light is a single purpose thing, can't help you with the rest of your system, even if you do trust it.
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