If you heard no distortion before the tweeter blew then the amplifier is not to blame. The speaker's tweeter just failed. If you had heard distortion it would have meant the receiver was being driven past it's design parameters and that could cause tweeter failure from OVER powering the small tweeter. It's also possible that you mechanically over powered the tweeter, but unlikely. The Sony speakers are not built to a high standard and it's most likely it was a weak part that failed under normal conditions, this is not impossible in the least so it is worth considering the possibility.
I understand that you like your speakers, but if you want better sound quality (clarity) then you will need different speakers. Better speakers don't have to be bigger and they don't have to go louder, they just have to be a good speaker. Most speakers would be a staggering improvement from the Sonys that you presently have.
You are under the assumption that you need more power because a speaker has a specification that says "power handling 120 watts" or what ever it might say. To give you a better understanding of how power and speakers relate I will use the specifications of your speakers and the assumption that your Mitsu is 50 watts per channel to illustrate how they interact with one another on a basic level.
Speakers have a specification called Sensitivity. The typical standard of sensitivity measurement is based on that speaker's output given input of 1 watt with some input device (microphone) or ear at a distance of 1 meter from the speaker. The sensitivity rating of the Sony speakers you have is 87db @ 1 watt/1 meter. When fed just 1 watt your speakers would be as loud as a hairdryer or power head on a sweeper. To determine how loud the speaker would be if fed more wattage you can easily determine by using a simple mathmatical method. For every 3db increase the wattage must double. In order to hit 90db you would need 2 watts, 93db - 4 watts, 96db - 8 watts, 99db - 16 watts, 102db - 32 watts, 105db - 64 watts....etc
So your speakers would probably hit a maximum level of output of around 103 - 105 db depending on the frequency being played which is almost as loud as a chainsaw. If you ran a chainsaw in your apartment I'm pretty sure your neighbors would not appreciate your doing so, so chances are you probably won't be attempting any 50 watt rms scenarios when your neighbors are home anyway.
There are some very nice options in the sub-$200 range for speakers. I suggest you have a listen to Klipsch and consider BIC speakers if you like your "highs". Both manufacturers have a trend of producing naturally bright sounding speakers that would probably be music to your ears.
Upgrading to another receiver will not make your speakers sound any better unless something is terribly wrong with your current receiver (like loud humming through speakers when no music is being played, over heats, has functions that don't work ....etc).
