Hi !

I'm on the way to change speakers on my 9494, cause they seems to be a bit old (little moisture scarves and weird little marks on it also when i Heard to it, it seems that the Amp is powerful but speakers can't handle it ( especially basses, treble and medium are quite good) that's why I want to put new one (just the big pair, I'm not going to change Tweeters , they work pretty good)
And I would to know if there is things I must know to make a good choice (of course I will look for same size speakers, with 8 Ohm impedance, but most of them are 8 Ohm'ones today)

you are right on that one and remember the behind clearance.. ie magnet, its good a slim style car speaker and yes it makes a big differance when you change speakers, i did my jvc rc-550 and replaced it with a sony 10in speaker rated at 20 watts fitted like a glove and boy she sounds terrific it came alive comparing to the original speaker that it had

There a a few things you should consider...

Take note of the impedance rating (usually 4, 6, or 8 ohms, etc. and how tweeters/midrange (if so equipped) are wired to main speakers. Best practice is to replace with matching impedance values in order to protect the amplifiers.

If possible, swap speakers with a doner boombox ( a beater, or one beyond repair, but still has good, working speakers). Chances are, they will more closely fit, and match electrical characteristics of the originals, and they'll be easier to find.

Check that mounting screw hole patterns match the originals, as well as physical size of the speaker so that they don't interfere with circuit boards and/or motors, or other moving mechanisms.

Try to resist the urge to swap originals with readily available car stereo speakers of same size...due to the usual hi power rating of most car stereo amplifiers available today, manufacturers tend to produce inefficient auto speakers, compared to boombox speakers which in most cases are highly efficient (to take advantage of normally lower powered amplifier output).

thx a lot for all these informations !
In Fact, I Think I won't be able to find a doner bbx easily so I think I'll buy new ones that's why I have another question about this:
Especially for the 9494, do you know if it's better to mount bass dedicated speakers or more "wild range" speakers (bass/medium)?
I think the second way is better (cause it's a 2 way system) but I never done it before on a bbx so I don't really know.

You will need a mid/bass speaker with a frequency rating up to at least 2500 to 3000Hz since this is usually the range the tweeters are designed to start rolling off. A full range driver would work fine but a bass only speaker is likely to leave you with a frequency response gap in the mids.

What ever you choose remember that the higher the efficiency the better since these boxes have relatively low powered amplifiers.

Follow the advice of JoeCool as well, he has listed some good points to be aware of.