To have a better control of the bass sound, I've improved my system to a multiamp.
I've built a solid state amp to use for the bass (Gainclone) and I use the 300b amp (theDream) for the medium and the treble.
Of course I've built an active crossover (12dB / Octave Linkwitz Riley Crossover, a project of Rod Elliott) to separate signal and i've made a little modify to the passive crossover.
The result was very satisfactory, because every amp works better for the right frequencies and the passive crossover is simplified: bass now works as I wanted.