Speaking of bad-sounding Marshalls... A story from an early band of mine. (Might belong in a thread titled, "Best Amp I've Ever Heard/Played Through" though, haha)
Other guitarist played a Super Reverb, but as a huge Jimmy Page fan, wanted a Marshall. This was back when we all simply referred to what are now called 'Plexis', as just 'Marshalls.' He
really wanted that sound! (So did I.

)
He found a good deal on a late 60's/early 70's Super Lead, 100 watt head, and shows up to a gig with it. Didn't have a Marshall cab, so he plugged the SR's speakers into it, and..., it sounded
terrible!
He goes to return it the next day, and long story short, he learns about Celestion speakers. Buys 2, puts them in a Dual Showman cab he had, and there it was! THE sound. In fact, it's probably the most unique-sounding Marshall amp I've ever heard, most likely due to him using that large cab with only 2 speakers.
I heard it many years later, and it sounded dull and lifeless, so I told him it likely needed new tubes, but he couldn't hear what I was hearing, no doubt due to how things like tubes and car shocks wear slowly, and one may not notice it, until you get them replaced. Which he finally did, along with a total going-over by the tech, and he swore it sounded better than ever. I bet it did.
He uses it with a Marshall Power Brake, which sounds great, but I had it at my house, alone one weekend, and cranked it up without it, and that thing is a fire-breathing dragon!