My new Accuphase 408 is lacking ......

If it's brand new, it probably needs some "burning in" time. Give it about 100 hours of playing, probably will sound much better.

How new is the 408? that would be a starting point to help with some possibile solutions.

Well I have asked the dealer in the UK, and he said it does not require a long burning in period(they suggested 10 hours), do you think I should change the speakers??
has any one tried the integrated E 408 with Proac D25 ??

what AC cable are you using? I was somewhat disappointed with my $10K Accuphase C275 pre until I began experimenting with upgrade cords. Settled on the Synergistic Research Designer Reference Squared AC X-series with active shielding (I think they call it the Absolute Reference now?). Esoteric equipment deserves esoteric cabling anyway.

I would still give it more time to burn in and settle before you start to change (even though I agree with (Bob_bundus), it migh be a "synergy" mismatch for you. If you decide to change ac cable as always if you have a chance to audition first it would be even more helpful, but try your more budget oriented change before a "heavier one" there's a lot of variabiles you're still working with. good luck and keep posting if you don't find it right off.