Testament - Low (1994)
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I knew the band from my heavy pals, older and more versed in thrasherisms, but I just thought the cover was ultra cool. Bought the cd and enjoyed the music a lot.
Woodentops - Giant (1986)
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I knew nothing about them. I was about to buy a ton of records from a discogs seller and the "add another item at no aditional cost" message showed. Checked his stuff and saw this NM for like 2 or 3 €. Didn't even take a quick listen on youtube, I literally thought "with such a good taste music must be at least decent". And it was.
These are the first that come to mind, there are a few more.