Advise on whether to upgrade to 440, Astra or something entirely different

[ray1234]

If 'vocals' sound thin, then just a subwoofer alone cannot fix that.

Good subwoofer, properly integrated, can do much more than just improve the bass.
And yes - it can affect vocals a lot. It can make them bigger, more pronounced, more forward (in the scene), with more "air"...

It affects the sound (the whole frequency range) much more than any cable, footer or similar stuff. Not that cables are not important.

I know subwoofers can affect vocals, but one is not supposed to use it that way. If a subwoofer is used to make vocals 'thicker', then it will have other unwanted side effects, like making the vocals less transparent.

Subwoofers when properly tuned should make vocals more '3D' holographic, but it should not be changing the thickness of the voice. Subwoofers will also make treble sound better and more airy, and the whole soundstage more holographic. It can increase density of the soundstage, but it should not make image bigger, if image is bigger and more forward, one is tuning it incorrectly.

Of course, I am referring to main speakers that at least goes down to 50hz +-3db, if it is really small main speakers that roll off very early at say 80hz, then it is a different story as the main speakers itself is not even able to cover the vocal frequency range and a subwoofer may compliment the vocal range.

[Jean-Marie]

A subwoofer can have another benefit, provided you properly do a high pass filtering of the main speakers: it removes from the main speakers the burden of low frequencies which are the most demanding and usually create inter modulation distortion.

Therefore as said by @Delija a well integrated subwoofer can improve frequency ranges much beyond just the lows.