I think I would trust Mr. Hafler before MC. Just my opinion.
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You got a plausible explanation for the capacitor from Atmasphere. I wouldn’t second guess him, but I do wonder why Hafler chose to use 10uF there and why he didn’t use a film cap, as well. Is this an MM stage or MC? If MC, what is the value of the load R? (“MC” defined here as moving coil, not the guy you asked for).
I looked up the schematic. C2 is the top half of a capacitative voltage divider network. C1 is the other half. Together these act on AC frequencies delivered to the input transistors. R1 is your phono load resistance. I have never seen this topology before, and I don’t quite get it. But Hafler was one of the greats. Don’t mess with it except maybe to upgrade the capacitors. Do NOT change their values.
I couldn’t download the article without paying a fee. However it’s been referred to in several ways at various sites. Seems C1 and C2 are tantalum in the OEM circuit. Tantalum’s are polar and correct orientation is mandatory lest they catch fire. (I know from first hand.) They are ok sounding but a modern electrolytic or a film cap will sound better. However tants are tiny and there isn’t much room for anything much bigger. Calls for some ingenuity. Suggestion to the OP. Before you remove a tantalum capacitor mark a PCB for polarity, and if you are replacing the tantalum with another polar cap like an electrolytic, conform to the polarity of the tantalum.