NB I have no link to Duelund products.
As expected the thinner the wire cross section the very low bass is not so prominent.
However I was very surprised at the change in sound.
But this was not exactly as expected The 2.5mm speaker wire is warmer sounding and quite revealing across the audio band.
The smaller cross sections give clarity to mid band and can be quite a surprise at first listen.
Obviously the smaller the cross section then tends to highlight higher frequencies, not that surprising after noticing the same changes on interconnects. The most surprising thing is again we are talking short lengths between speaker terminal on the amplifier and the output transformer. Its always a YMMV but interesting none the less.
Many will just think a small difference but it isn't, its as much as a coupling capacitor change.
What I would mention is strip the insulation using a sharp knife "carefully" or you will break the wire strands. Also the only way to get the 2.5mm through the transformer ring tags is put it through without tinning then solder or you won't get it through.

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Its interesting to note that the "Warmth" of sound is linked mainly to the 2.5mm the smaller size tends to give the amp a more focused image. With a size down in wire the bass is still there its just not so warm sounding "strange".
So does it make a difference unfortunately yes

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NB I used a small band of heat shrink on the end of each lead close to the end of the insulation it gives +/- indication (red/black) but it also ensures the cotton insulation cant unwind over the years. So the heat shrink is just a 4mm band over the insulation not in contact with the conductor.
Regards
M. Gregg