Many certainly were as the fix to the circuit (especially of the Stereo 20) is well known. As I understand it there were two ‘design errors’:
a) initially the amps ran the then recently introduced EL84 valves right at the edge of their envelope, and only Mullards could take it, other brands just blew up despite being in ‘spec’, this was fixed by changing some resistor values and eventually the output transformer was redesigned.
b) the Stereo 20 uses pretty much the same mains transformer as the mono TL12 Plus and once it was partnered with an also twice the circuit in the new stereo preamps, exported to Australia or other hot climates and stuck in a cabinet they overheated really badly spewing wax everywhere. The transformer was later changed.
My understanding is a lot of both amps ended up back at the factory or national distributor as warranty repairs, but after that it is anyone’s guess. These amps are the very definition of ‘Right To Repair’ being easy to work on and coming with a full schematic and layout map in the manual, which is one reason so many are still serviceable 60+ years on!
One problem is the schematic that shipped with them often doesn’t reflect the actual amp, e.g. my Stereo 20 looks to have been shipped with the later more gentle resistor values though the manual is of the original amp, i.e. almost certainly was ‘wrong’ from new. My suspicion is both my TL12 Plus hadn’t ever been back to Leak as they were still running the more aggressive (1M) resistor values though the each have had some minor work done (a capacitor on one, cathode bias resistors on the other). I’d obviously love to know more of their story, but that is lost to time. I don’t even have birth dates for these, but judging by the supplied valves, some of which may be original, my guess is they are both from 1958 or thereabouts. My bronze Stereo 20 still has it’s sign-off tag so I know for sure that is 1961, and I suspect the grey 20 is 1965 as it had a lovely test as NOS full Blackburn Mullard valve set dated 65. Matching date codes on both the quad of EL84s and the trio of ECC83s.