Narry a problem.
I don't have this exact combination, so I can only approach it from a theoretical standpoint. Thing is, speaker cables is a bit of a hiding to nothing because recommendations must always be taken with a large dose of salt. Wire is very much a passive component, and I have yet to see some evidence I can swallow without gagging that it has any intrinsic 'sound' whatsoever of its own. What it certainly does have are electrical characteristics which affect the performance of the components it connects together -in this case, an amplifier and speaker drive-unit. So unless a person has the exact same speakers & amp as yourself, any suggestions must be taken with this in mind. It's just the application of physics, no more, no less.
FWIW, I suspect that adding a pre-amp is probably countering the resistance of your wire. You'd likely get similar results by swapping out your wire for something wth lower resistance. 0.4mm is about 26AWG, and that's very thin indeed, as speaker cable goes. I've used 30AWG in the past, on a low-Q unit with a low output impedance amp, which is a useful trick, but ideally, you want to be matching your driver / speaker to your amp. That way, you don't need to resort to using wire as a tone control to rebalance the sound. I suspect your Almarro amps & 208s would sing happily with something like 14 - 10AWG wire in place, & no active pre required. Cheap experiment -grab some mains cable & give it a shot. Shouldn't cost more than a few £. :^)