Danelectro Danecho DE1; 2 units both with IC4 failure

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I've just repaired a couple of these DE1 pedals for a friend; he liked the first so much he bought another when the first one died.
In both cases IC4, an RC4558 dual opamp had failed.
See attached schematic that luckily an internet search picked up; amazingly, 'Basoni' must have reverse engineered and drawn up very professionally. It seems accurate.

IC4A amplifies the already buffered input about 10dB and sends it off to the main chip etc, IC4B mixes the wet and dry signals and sends them off to the output.
Weirdly (to me) the output is loaded with 470R R201.

With one unit, IC4B only had failed, with the other both sections were dead.
Fixed, they do sound good!

Is there a design flaw? If so any thoughts on how to make them more resilient?
All I can think is ESD, and maybe add a small series resistor (eg 100R) and couple of series pairs of back to back diode clippers across the output.

[Enzo]

In most circuits, the part that interfaces with the real world is at highest risk. IC4 drives the output. SO if I had to predict the most often failed part in one, I'd pick that chip. Not because the chip is under spec or unreliable, but because who knows what it encounters connected to that jack. Well really, I suspect that IC after the power input stuff. People blow that up all the time, "What, adaptors are not all the same???"

They made tens of thousands of these things, and I would not be ready to claim design flaw because two of them had a failed chip in such a position. In my view the output killed the IC, and in the one with two bad sides, it only takes one failure to kill both halves. What does this guy connect the output to?

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He uses it plugged into the main instrument input of a couple of regular tube guitar amps, either an old Sound City or a fairly recent Fender, Blues Deluxe maybe.
An old Boss echo used for a long time before these, and again now, seems to survive ok.