The speakers that replaced the factory ones may have been of a lower sensitivity. Sensitivity means nothing really if you have sufficient power available, some people go for high sensitivity speakers for certain reasons, but in this sort of case it doesn't apply.
Just because you put a better speaker in, doesn't mean it will sound any better. If you haven't changed the source, or provided any extra amplification, then what do you expect? The speakers are the final end of the chain, you need balance in the whole system, not just in one area.
300w means nothing, it's also it's peak power output, not it's nominal output, even a speakers nominal output or power handling often has nothing to do with how "loud" they will be. Bottom line is you get what you pay for, you didn't spend much, and you only changed part of the system. If you want any further help you will have to at least provide a model number of the components you put in. You may have even wired something out of phase, or something simple like that.
With regards to line out convertor and amplifier, you take the signal close to the head unit so it's before the factory crossover parts. Speaker wire goes in, rca cable goes out. rca cable goes to amp, speaker cable goes from amp to new crossover, from crossover to speakers. If you bought coaxials you may not have a crossover unit to worry about. Anyways more info is needed.