A 4x6 speaker is by the nature of it's small cone area going to be limited in output. I would look into the Infinity Reference 6422cfp 2-way 4x6 plate mounted components. It's not going to thump but it's the nicest 4x6" replacement I've seen.
i'll second those Reference plates. if you want more volume, Infinity also makes a Kappa version. but they need to be crossed over around 250Hz and you need a dedicated midbass driver.