Hi there, I found the same thing. I changed mine about 18 months ago. The description i got from the bloke who fitted the new ones was those lod ones were shagged, especially the left one. lol. I have some old speakers in my car that I bought for my mk2 1.4
ibiza sxe that are now in my mk4 they are fairly decent. They are Alpine SPS 1757's I think. They are 6.5 inch speakers or 17cm which ever measurement you like. To fit any aftermarket speakers you will need speaker adaptors. The rear quarters are exactly the same so if you want to change those, again the speaker adaptors are required. I think car audio direct sell them but you have to look at the previous model to find them. Im not certain these will do the job on our mk4's. But a local
shop that fits speakers and stuff should be able to order them in for you.
I also fitted new speakers in the rear quarter which were Audiobahn AS62v they are pretty good. They are rated at 90w rms which is what you want to go by, not the max wattage. I've wired these and some old alpine 6x9's up to a JBL GTO 1004e four channel amp which is rated at 4x100w rms. Myself and others who have been in my car have all liked the sound.
Any alpine, audiobahn speaker should be good. There are also many others that are good, like Kenwood, JBL, Sony and Pioneer. Just depends on your budget. Really you will find most aftermarket speakers will sound much much better than the originals as long as they have reletively decent rms wattage and max wattage you'll be able to push the sound higher with better quality.
Hope that helps.