There's two very different things I'd consider and am currently setting myself up to test:
Aging Capacitors
(Bracing for the Capacitor police and the Anti-Cap-italists) Aging capacitors can affect performance both in the power stage and sound path. Weak power caps wont be able to provide the power reserve needed for dynamic and punchy bass. Weak caps in the sound path can affect frequencies, which area of the spectrum affected will be dependant on so many variables (circuit design, cap values, make, model etc.) I couldn't be more specific.
Speaker enclosure mods
If your PH-W405 speakers are anything like my Sony CFS1000's or JVC PC-W150's then the reason they have clips to secure them to the stereo unit is to stop them from floating away! Seriously, they're so light weight the speaker cables act as tethers to ensure a light breeze doesn't whisk them away. Now being a portable stereo it's understandable why lightness might be high on the list of requirements but it does mean that sound quality will likely be affected.
The reason why quality HiFi speakers are (usually) heavy is because a speaker builder is looking for a stiff and harmonically inert enclosure material and mass plays a large part in that. For this reason I'm planning to mass-load the enclosures by cutting templates of the inside of the enclosure out of card (e.g. cereal box) and using them to cut out the shapes from vehicle sound-deadening material (e.g. a cheap version of Dynamat), line the speaker box with this, brace with MDF if required then stuff with speaker fill (aka the insides of an old synthetic fibre pillow) and add a textured foam to the back of the enclosue in an attempt to reduce reflection, and therefore cancellation of, specific sound frequencies.
I expect the result to be a "fuller" sound with better bass and overall more "presence" but only by testing will I be able to confirm this.
Annecdotally I have in the past greatly improved the performance of small PC and surround sound speakers just by opening them up and adding speaker fill (pillow stuffing).