The chassis of an audio component isn't just a box. It's part of the circuit, and definitely part of the sonics.
Electronic components are mechanical things as well as electrical ones. Capacitors flex slightly under their own dielectric stresses. Transformers vibrate at line frequency. Tubes are microphonic — physical vibration of their internal elements directly modulates the audio signal passing through them. Printed circuit boards resonate, and those resonances couple back into the components mounted on them. A chassis that resonates sympathetically with airborne sound from the speakers, or with vibration coming up through the rack, feeds those motions right back into the circuit it was supposed to be protecting.
So a well-designed chassis does several jobs at once. It provides shielding from radiated electromagnetic noise. It serves as a low-impedance ground reference. It mechanically damps internal resonances. And ideally it isolates the sensitive electronics inside from external vibration that would otherwise modulate the signal in subtle but audible ways. A flimsy chassis can compromise an otherwise excellent circuit, which is why serious audio gear has always been built heavier and stiffer than the strict electrical requirements would suggest.
The old cliché about audio by the pound isn't too far off the mark, but usually for the wrong reasons. The weight by itself doesn't help. What helps is what the weight indicates—a chassis built to control rather than transmit vibration. A perfectly designed lightweight chassis can be just as good, but it's harder to do and rarer to find. Mass is the easiest way to get there, and that's why most of us have learned to trust gear that's hard to lift.
The box isn't the show.
But ignore it and the show suffers in ways you can hear but never quite name.
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