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So, the question is, should I use headphones or continue with my Stage Pa300 speaker system?
It depends. Here's my opinion...
Headphones are to be used for sound quality analysis. By having sound source practically in our ears, we can hear every so subtle detail of each instrument. Not because headphones have better quality sound, but because sound is totally isolated from environment disturbances (noise, echo, sound phase shifting, etc). In short, with headphones we can judge about instruments quality (simplified: $50 vs $1000 guitar).
But if we want to know how the ensemble (band) will sound on stage, external speakers should be used. Let me explain... headphones have a big drawback: with headphone, stereo "stage" is flat. All sounds are only between left and right ear -there's no "in front" (no depth). Our hearing is made in the way, so we can determine direction of sound source (because of ear shell). With headphones, ear shell has no function and stage depth is lost.
The second reason why using speakers is low frequencies. Yes, many headphones can play sounds all the way down to 20Hz, while we can be happy if our speakers can go below 50Hz. But, in real world, we don't only hear low frequencies (i.e. bass guitar).. our body also feels vibrations -and that doesn't happen with headphones.
That is also the reason (among others) why every studio also uses near field monitor speakers. But at the end, everyone should do what works better for him.
Bogdan