Thank you for your answer. The problem for me at this time is that I can't build active filter. For the future, yes, I will build a tube crossover and a mosfet amplifier for basses. But at this time I can only use a passive filter or replace the Delta 12LF by another 12" wich can be work in a 100 Litres box.
I have read on the net that at the resonnance frequency, the power fall down with the peak impedance. For a driver wich have 6 ohms at 200Hz, and wich have 45 ohms at Fs, if you have 100W at 200Hz you have only 13W at Fs. So it certainly my problem with tube amp. I have tried my loudspeaker with a solid state amplifier (yamaha of my home theater system) and the problem is not so perceptible/discernible.
I have found a link to calculate series notch filter on the net, but I'm not sure of the quality http://www.mhsoft.nl/SeriesNotchfilter.asp.
Do you know this link ?

Thank you for all

Cordialement.

Pascal.