I'll bite.
Marshall (from SLP to JCM)
Now, it's not that *this* specific amp itself is overrated - on the contrary - they're fkn marvellous. But E V E R Y T I M E I read a thread about NAD or this amp or that amp - the ever-ubiquitous question "can it do plexi??" comes up - and every time I read this, I'm like "just buy the fkn Mashall to start with and stop trying to make whatever unobtainium-priced booteek amp ya just got try to sound like one".
The reason so much of the heavier music from the 70s sounds similar is because for the most part, all the heavy bands were running the same amps by Marshall, and often playing humbucker loaded Gibbies; if not Gibbis, Fenders - note how Malcolm Young had a unique tone - same backline as his bro, but unique guitar. Back then, the players guitars and technique dictated the flavour of tonal difference (ie: listen to the opening of "Coast to Coast" (Scorpions) and whatever similar era ACDC song - for the first few bars, indistinguishable). Then the modders came along trying to make the Marshall template unique from the pack, and some of these modders turned to making their own amps. Some of these latter mentioned amps are very unique - completely "out there" from their perhaps original Marshall DNA roots - but that's the point. To sound different. Not for some TGP buyer to fap and faff about some booteek-overpriced crown jewel amp being able......to do "plexi" or "crunch".
Blows my mind. Without fail.