I'm no authority.Speakers are 8 ohms. So stuck to 16 AWG. But still will try as the suggestion comes from a senior authority like yourself.
Regards,
Sandeep
Try to get 2.5 sqmm OFC (4 sqmm if you have banana plugs) wire from companies like Polycab or Finolex or similar.
The shop will try to sell you a spool, but if you have done business with them earlier, they'll oblige and give you cut length.
If the cable experiment works for you, problem solved (for now), until you start picking on something else
If it does not work, Rs. 200-300 loss and you eliminated one possibility.
Then you can start your pursuit of component change, etc.
Cheers,
Raghu