Amping a set of aftermarket speakers will usually give you clarity and gain some midbass. A clean signal will aslo help so most people start with a decent head unit, switch to aftermarket speakers then add an amp, then add a sub and amp.
If you want to simply change speakers, you'll need to go somewhere and listen to brands. Look for speakers with a higher dB rating and they'll sound louder. Keeping with 35 watt rms speakers will be better if you don't have an amp, if you have an amp math the rms rating to the amp's rms rating.
iPhone app user; excuse typos...thanx.