I believe the Ply 3.4 is 12awg and the Classic 6.0 is 9awg.
It looks like your Merlin are 8ohm nominal, going down not lower than 6.5 ohm... An easy load. So you may not benefit from the lower resistance of a mega large wire like the classic 6.0, unless your length requires it.
With the ply 3.4, you have more than adequate awg and you have a lower inductance than the classic because of the cable construction... I would stick with the ply, it looks like it is the better cable, but hey classic 6 is less than $4 a foot, so it may be worth the try, and your sound will possibly change if that’s what you are looking for.
It looks like your Merlin are 8ohm nominal, going down not lower than 6.5 ohm... An easy load. So you may not benefit from the lower resistance of a mega large wire like the classic 6.0, unless your length requires it.
With the ply 3.4, you have more than adequate awg and you have a lower inductance than the classic because of the cable construction... I would stick with the ply, it looks like it is the better cable, but hey classic 6 is less than $4 a foot, so it may be worth the try, and your sound will possibly change if that’s what you are looking for.