These little tweeter speakers in my experience are about as much use as a chocolate teapot in a vehicle, two in the door pillars of my lorry made horrid sounds, now muffled which has improved things greatly, i suspect switching your tweeters off would improve things too.

You've got 4 x50W output from your head unit, so my suggestion (and yes do check for adequate mounting/sealing/baffling of the current door speakers) would be to fit a suitable pair of at least 6" in the rear sections inner skins and to disconnect the tweeters...indeed if removing (or muffling) the tweeters improves things you could maybe find better speakers to replace those and wire them as the front set.

An alternative, or addition, could be one of the modern small active subs now available.

The family Aygo is similarly set up to yours, but Sony headed, 4 x Alpine speakered not as there's any great difference, but the dash speakers are approx 4" 50W RMS each as against tweeters. I also fitted a Pioneer under the seat sub (other makes available) which has put the icing on the audio cake.

On each part of the change i tried the sound, this proved that every bit was needed as there was improvement each time.

You've obviously already been through the head unit menu and set it up with Loud etc enabled.

Is there any chance that you have crossed the +/- on either of the speakers?

Having one of the speakers wired opposite to the other will have a big impact on bass.

Sub is a doddle, simply cable tie it to the carpet under the seat for ease, then wire up same as any active sub.

Power wire direct from battery (preferably with fuse as close to batt terminal as poss) through a convenient gromit throgh the bulkhead, switching wire to the blue wire connection of your head unit, then connect the two main connections to the pre outs found on the back of your head unit, job done...note some subs come with all wiring supplied some don't.

As its a Berlie with hopefully some decent under seat space (or other handy space), you might be able to put a couple of cheaper small passive low power subs in there, (or build some 9" round speakers into home made MDF cabinets made to size) one under each seat and simply wire them to the normal rear speaker outputs instead, basically just using them as rear speakers, endless possibilities.

Wish my W124 Benz was as roomy for audio upgrades, very difficult to do much with as the maker saw fit to put small less than 4" speakers in the dash top (remarkably quite good) and similar rounds in the rear parcel shelf in purpose made cabinets, even half decent correct rear upgrades run at £150 a pair, still only 5" jobbies, there is one particular Pioneer speaker which given a litte trimming and fidding with does the rear job nicely for about £70, but a dickens of a job to find them.

Did have a proper sub in the boot to give the system some bass, but with the LPG tank in there it was getting full, no room under seats so have put an under the seat active in place of the first aid kit in the rear parcel shelf, not ideal but much better than the very poor standard sound.

That would have been my next comment,ie manufacturers cable usually is made for about 10-20 watts music power per speaker....for 50 watts(50 volts) it needs thicker cable

My other point is if the ohms resistance of your radio is 8 ohms but your speakers are 16 ohms you will lose approx half your volume,it also works the opposite way round as well except you reach distortion point before half volume

I dont wish to confuse things but RMS power output(equivalent power in volts) in my opinion should be given on all units sold instead of music power and total output as they dont give accurate discription of real sound output,speakers should be labeled the same imo

Metal oxide transister they are power amp transisters supposed to give high power with low distortion,amps i`ve had with them have had very good sound quality not sure of new models

I think you know the solution to your problem then...

I wouldnt worry about the cables untill you get the volume sorted, standard cable should be fine up to almost half the output.

I cannot find what the 50watt is JVC dont specify so I can only assume its actually about 12-15 rms,does it say what ohms speakers to use with it?

Sorry but you dont need high quality speaker cable in a motor unless your spending a fortune on hi fi,oxygen free cable isnt worth it, only for a home system because you wont hear the difference,1.5- 4mm standard cable is good enough even mains cable will cope with the peaks of voltage...

But you still have to sort out the resistance mismatch which the cable wont interfere unless the volume is very high,once that is sorted by all means put what cable in you want

speakers should be 4 or 8 ohms you can find some are 6 but its nominal impedance ie it fluctuates, personally I would fit 4 ohms but thats me, you should then get the best from HU

Some are 16 Ohms but will halve the output due to poor resistance,you could have volume flat out but it wont sound it,if you have the correct speakers already it can only be low output from head unit or as stated cable but I`d go for former

Good luck with it, you would be surprised how many people have problems with in car ent