I have the same 6x4 in the front door of the Coupe. I looked into replacements and could only find two options in the UK. Both were big brand but fairly bleah in both sound quality and power terms. I did not bother changing them in the end.

If the S5 is like the door of the Coupe/S2 the depth of the mount is an issue so you cannot get a round+deep speaker without a lot of work and non-flush mounting. The depth means you cannot easily fit a large magnet high power speaker. It will also impede your foot path getting in and out if you go for a raised mount.

I used the amp in the head unit to power the fronts and then bolted a better set of 6x9 Focals in the back to replace the factory upgrade ones. I put in a 2 channel class-d amp for the rears to try and control them. To be fair the side exhausts and dB level in the hard top Coupe means I am fighting a loosing battle so I just accept it.

Whilst you are in there!

I would recommend looking at the cable path from the dash through to the door. This is normally VERY tight so replacement speaker cable to improve that is not an easy option. Do make certain the wire protection through the body/door opening is solid or the wires will short leading to full-on sleepless nights.

Everyone loves a Morgan. Even me, unless it's broken again.