Lionel said:

My problem Arnold is that I don't really like the sound of "modern"
speakers because I like quick and precise answer from the bass an not
this diffuse and not well defined loud bass that we can hear from a lot
of speakers and moreover from a lot of subwoofer.
In fact I just need to reproduce with the maximum fidelity the sound of
a double-bass.

Before you proceed with your speakers, you should *definitely* have a
chat with Mattijs de Vries at Machmat.
He seems to have exactly the same thoughts about reproduction as you
do.
He could keep you from making expensive mistakes.
Shoot him an e-mail, he's a very friendly guy. His only drawback is
that he talks too much when the subject is horns or tubes .........
Oh, and don't try to write in French, he'll understand NONE of it :-)

I have learnt about this issue. I was thinking at a transition with a
simple 0.47 µF cap according to Fostex recommendations (perhaps a Jensen
one in order to give a little bit of blue blood to my speakers) ).

Jensen PIOs would be a waste of resources in loudspeaker filters.
They're used best at lower level AC signals with highish DC
potentials, such as coupling caps in a tube amp.
I wouldn't even use them in a solid state amp.
IT (Intertechnik) makes very good affordable MKPs for that purpose, as
does SCR Chateauroux (SP?) and Auricap. At least IT and SCR should be
available to you in France. If not, shoot me an e-mail and I'll get
you some addresses.
Because of the lower voltages involved (mostly 100 V max) they'll be
cheaper than dedicated tube amp capacitors with higher voltages.
They sound different, too.

--
Sander deWaal
"SOA of a KT88? Sufficient."