Improvement on acoustic box

Hi, a few days ago I bought a pair of Technics SB-3130, and although they sound pretty well I'm not completely convinced by their sound.
Mostly I notice some muffled bass. So, what I was looking for was some modification to the box, for example, adding acoustic filler or making a hole like bass reflex on the back side.

Would it improve the sound or would it damage the speakers?
I accept ideas like that.
Thank you very much!

 

Modifying these speakers is not a good idea, they are already designed by Japanese engineers as a complete system. My guess is that they do not have the original woofers (with the inverted surround on the woofer) as the original woofers surrounds had a tendency to deteriorate over time. If the woofers are not original (search google images for examples) then you're going to need to determine if the woofers are simply repaired (new surrounds) or have been fitted with new drivers. If they are fitted with replacement woofers, you're out of luck unless you replace them with original. If the woofers are original but have been repaired, you may be able to find a speaker repair shop who can correctly repair them with the correct surrounds and the correct impedance (ohms.) As the crossover network is designed for a specific ohm load if a replacement is installed with a different impedance the crossover point changes. If the compliance and all the rest of the Thiel Small parameters are not followed from the original design, you're wasting your time attempting to try and get these speakers to sound correct as they were designed.

 

Okay, first of all, thanks for your help, I've done what you said, I've researched for some information and disassembled the drivers. They seem to be the original ones, or at least, they've been replaced by some identical to them (Technics inscription on every driver). Also the crossover seems stock with Matshita electronics in them.
How're you so sure that the woofers on that speaker were originally made with inverted surrounds?, because on none of the google images I saw look inverted.