More first world problems, weight Vs tunes?

Bit of a quandary, Temeraire is currently fitted with some nice but standard size speakers which with the eq set correctly are not without merit. However there's a limit to the lowest frequency response to expect from a 4 inch cone. To that end I'm thinking about 6x9s in the rear deck foot well behind the seats. These would be full range and I would high pass filter the front speakers with a passive, and adjust the low pass on the sub to stop it working so hard. I'm trying to get my head round the weight implications of making a suitable enclosed box for the 6x9s as I would want them sealed for the music I love. This could eat the 20kg spare room I have especially if I find I need to amp the 6x9s but that's mission creep and I'm not there yet. This also leads into finding a 6x9 that is happy with 60w RMS to avoid an amplifier?

I must admit I am not 100% sure what you are asking.
To me reads like you don't want to add more than 20kg to "box in" the speakers, but then I don't understand why this is an issue unless they are on the roof rack
Sorry if I am being more confused than normal, just trying to understand your wants on the off chance I can help.

For clarification in its current form this Jimny is 20 kg lighter than the last one which is great for both payload and not making a mess of the land on the farm which ought to be the main priority.

However while the stereo is currently not bad it starts to show its limitations when you push it beyond half volume. To a normal person that is not a big deal. I'm not normal. Music is not loud until you can feel it in your chest. And that's my issue is trying to reconcile the 2.

Many years ago I adopted the position that attempting to create a "HiFi environment " in a car was more headache that it was worth. There will always be compromises, due to limitations that are in many cases, unavoidable.

Let's start with cabin size (which you cannot change) and it's dictates on speaker placement. As you've pointed out there is a limitation on the low frequency response you can get from a 4" driver, you mention that you're using "some nice, but standard sized speakers", which suggests you've upgraded, perhaps to a coaxial or similar, which would have the tweeters, more or less pointed at your ankles - far from ideal.

Another option is separates, with the tweeters mounted up on the "sail panels" - this incidentally is something I have considered, among other reasons I have an older pair of Kenwood separates with 4" drivers and 1" tweeters - this option however, has the tweeters physically separated from the mids, and causes a shift in sound stage, added to which one tweeter is considerably closer to each of the front seat listeners than the other, and these, as I suggested earlier are unavoidable situations.

I may yet install those Kenwoods, but for now all I've done is add a small active sub under the driver's seat to reinforce the bottom end, the car is not the quietest of environments, I have a transfer case whine, that maybe I'll try to tame by adding more padding under the transmission tunnel carpeting, but all in all, I recognize that I'm not in the car for long enough periods to put any serious effort into sorting it out.

Historically I have always had fully amped systems in cars that weren't for work and have only in recent years of Jimny moved to a high power head unit because of the space constraints. What I also have difficulty with is the local Tarquin brigade in daddies amarok being able to drown my none standard system, I find it an affront to my masculinity

Ok thanks for taking the time to clarify for me.
Don't think I can help, but 20kg is not much.

Can't you put the Jimny on a diet and bin all none essential items?

Do you need the passenger seat, how about a custom light exhaust, find  tyres that are lighter than yours or just drive with the tank half full, 4 gallons less is about 20kg. 
Really depends how much you want to save the grass or feel the music