When shown a circuit, you can almost always find fault with it. You ask yourself, "What's wrong with amp designers these days?! Can't anyone design competently?".
The pen is mightier than the sword, but not mightier than a soldering iron.
Your toilet has tube, DIY parts' catalogue.
Your cell phone's speed dial has at least 2 DIY gurus. And the marking on these keys to invoke the speed dial is blurred.
You have no problem remembering plate resistance, operating points of 2A3, 300B and 845, but struggle to recall your house phone number.
You want to name your daughter Svetlana.
You know more about the different dielectric materials in capacitors than an average Chemical Engineer.
Would cryogenic treatment turn your wife into a better wife?
You want to name your daughter Sylvania.
Your amp wears more silver than your wife.
You don't think there is anything wrong with amps that weigh 100kg, consume 60W of electrical power and puts out only 3 miserly Watts of music.
Your idea of financial security is a stash of Telefunken/Mullard 12AX7s and Western Electric 300Bs.
Your wet dream is finding such a stash!
You want to change your family name to Mullard.
Your monthly household budget spreadsheet has one column for DIY parts.
That's all I could think of. Send me your suggestions and I'll add to the above list.