An obsession with weight, in the absence of an understanding of what it may infer, led to Japanese manufacturers in the 70s and 80s to fit heavy plates to the bottom chassis of amps. This supported a sales technique of a customer to lift an amp to see how heavy it was and, therefore, how robustly it was constructed.
Today's version of this old scam is that heavy chassis reduce vibrations. That may be true to an extent but how often is this really a problem?
Weight means nothing by itself. But weight, with impressive specs (power supply current draw, scaled 8 and 4 ohm ratings, low operating temps denoting the presence of adequate heat sinking, etc might infer something.
If the person selling the gear can't logically explain why an amp is heavy and there's no obvious specs to logically explain it, then the weight is probably a meaningless design feature used to drive the myth that heavy = good.
If in doubt, ask the salesman to open the unit (which will almost certainly be refused at a big box store but may be done at a sound specialty shop if there's anything behind the claim). Some manufacturers make it easy by sending stores units with Perspex top plates allowing shoppers to see component layout, heat sinks, transformers etc.
