Got the combined phones socket and motorised vol pot board removed now.
Phones uses switched ground contact for cutting speaker output , via relays,
so not a
suspect now. Can see why there are "pot" problems with these amps but not
this one - YET. The PbF soldering on this board looks terrible compared to
main board , no chance of the "green ring" of concave sided volcano mirror
PbF finish here under a 30x . Matt grey crystalline and cracking solder so
the "ring crack" of PbF likely in a few years with normal localised user
induced vibration from using phones or turning the vol knob.

A technical issue - if anyone is up on the matter as twice in one month ,
different makers. I've found PbF solder being used on boards apparently made
from substrate not specified for PbF , or rather the extra temperatures/heat
reached in the soldering process.
From the UL.com E number and type number these amp boards are tin-tinned for
PbF use but the board material is rated 260 deg C/ 8second dwell (270 /10
for PbF , 260/10 for SnPb and 288/8 for polyimide high temp material ) so
lower spec than the already lower spec (8 second dwell time) of traditional
solder formulation boards.