Yes, the stereo is very average. It's a mixture of cheap headunit and cheap speakers. I'd change the head unit first and see if it works enough, then change the speakers if it doesn't.
Tyres: I no understand? Potenzas are made by Bridgestone? Best? Personally I'm with the Goodyear Eagle Asymmetrics crew, but some people prefer the Potenzas (RE070s IIRC). Depends on your preferred "feel": Eagles has softer ride, and slightly less grip in the dry (but progressive breakaway). Potenzas have a harsher ride, but better slightly better dry grip (but more abrupt loss of grip). Those are the only two I've tried.
Eagles are total pap on a performance car, so soft it's silly. I've been through this before and took them off, but last week I put a pair back on the rear when I was feeling cheap. Reminds me why I hated them before, I wasn't being irrational they really are like riding on jelly. Ruined the turn-in, rolls around like a barge, giving it the beans down a bumpy country lane had the rear wagging around as if the axle wasn't attached, really quite unnerving. I was going to switch them onto the front but even that was too much trouble, stupid soggy bulging sidewall would not clear the coilover. For now I have pumped them up to 45 and put 8 clicks more on the dampers.
The upshot of all this is that I found out JJC have imported some of the new AD08R and they are no more expensive than the vanilla model. Tempted to try these although I would be miffed if they turned out to not have the longevity of the regular ones.

) Hope that's some help?

