When soldering plugs to cables that's all fine of course, the contact transition makes for a slightly higher resistance but there's nothing you can do about that. In any case, make sure that the soldering joint is smooth and shiny. This is an indication of a proper weld. If the resulting solderjoint is frosted, this indicates a cold weld, actually not one nice connection but in fact hundreds of tiny succeeded connections and hundreds of non-connections. Use as little solder as possible and make sure that the wire you're soldering is as close to the tab as possible before soldering.
When using solder at the end of cable litze in order to make them fit into screw connectors more easily is a bad idea. Not a lot of people are aware that solder flows under pressure. This is a slow process but a very real one. After months of use the connection gets looser and looser. This is bad for the sound and also potentialy dangerous. This is why we never use solder in this fashion for our cables.
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