I've just changed the speakers and tweeters and I'm happy enough with the sound. There's one member thats Done a full custom install with a false floor for the amp and twin subs up the corners. It depends on what you want really. You can make anything better if you have enough ££££.
I reckon changing the rear parcel speakers may improve bass but it's having the time to try it.
I'd also check that all your speakers are working and haven't blown. Some cars have had the rear door speakers removed!
Just bought 2009 wersion of Honda accord, and I'm thinking of an audio upgrade in nearest future as well, here is what I found on ebay http://www.ebay.co.uk/itm/251653385578?_trksid=p2060778.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT so you could buy this double din adapter, and then go for any aftermarket stereo, but you would also need special cable if you don't want to loose the option to control from the steering wheel.