Well, mp3 is good for what it is good for.  Just not for everything.  Mp3 is a compressed file format, meaning it is basically shrunk down to make it smaller to send via email or to stream off a website or to just take up less space on your computer or media player.  The problem is, like evaporated milk, something gets lost/changed in that process.  Mp3s are called "lossy", because they delete some of the musical information that the encoder thinks isn't important. 

They are still good for general purposes, but they just have their limits.  Kinda like cassette tapes.

I use OGG format where you might use mp3 since it is what came with my Linux operating system. LOL  I feel it is a bit better than mp3, but since I still usually convert to mp3 for anything other people have to hear and I want to send as a file, it doesn't make much difference.  I feel it's maybe a bit better, and it is native to some of my software.

Now if you want actually better.. WAV files are not "lossy", and they save any music you record as much like the original as possible.  Better sound quality.  But they are large.

FLAC or on a Mac, ALAC, are compressed, so they are smaller on your player or computer, but they are "lossless".  Like the WAV, but smaller to store.  They are a better option for sound quality with a reasonable storage size.

Now MIDI... The problem there is that different hardware manufacturers and software companies don't actually agree on the pitches.  It makes it a pain in the butt, since you have to keep noticing it has happened and adjusting it.  Oh well, nothing is perfect.  Especially not MIDI.  LOL 

The problem you're describing, Oliver, sounds like it could be "choking".  What you did with turning off some voices to see if it sounds better, that's the diagnostic trick for seeing if a system is choking.  "Choking" is what we call it when you are running too many instruments and/or effects in midi for the computer to actually handle. 

That's the bad news. 

The good news is that you can just mute some voices or turn off some effects while composing or whatever, and then turn them back on if you are using software that can export/render the sound into a WAV or whatever audio file.  Basically play back all the sounds in full quality while the computer records them without making a sound through the speakers.  That way an older machine actually *can* do a full orchestra with effects on every single instrument and etc.  All it has to do to "render" is keep the math straight, not make is sound good through the speakers while doing it.  The end result can sound way better than what your system can do in "real time".

I'm probably getting a bit too "techie-talk" with all this, but I was just taking a course on DAW software and studio hardware the past couple months.  So I am even more "full of it" than I usually am.  LOL

Yeah, as Pierre said, MIDI isn't really sound itself, it is the instructions to your gear for how to play the sounds.  It's a lot like a written score that way.  Or a high-tech equivalent of a player piano roll.  How will it sound?  Well, it will always depend on the gear (using that word here for both hardware and software) that you play it through.

That being said.. It *is* possible for a MIDI file to also not be great.  Just like a score can be written by someone where you don't like their arrangement or version of a song very well.  And with MIDI, if their hardware was different than yours (almost always true), and they adjusted the pitches/tuning for their gear, it can sound kinda off on your gear. 

To give an example what I mean.. Imagine somebody is using midi with some voices on their computer/soundcard, but they also decide to hook in something like maybe a sound module or a synth with voices like a DX7 or maybe a sampler keyboard, because they want to do a flute part that sounds a bit nicer.  They keyboard probably won't agree with their soundcard on the exact pitch of the notes, so they adjust for that.  No problem.  Until you try to play the MIDI file they made on their gear on YOUR gear.  Then the parts actually are out of tune.

That's just one way it can happen.  I won't bore you with the others that come to mind.  LOL

MIDI is great.  Not the absolute best and most hi tech thing out there, but very useful and dependable for many things.  But for being something that doesn't take a huge super fast machine to be pretty useful, and that can talk to a really wide variety of gear, it is probably one of the most useful tools for someone running a home recording studio. 

I think that you might want to go with a wound, rather than plain E string for that, if you want the sound to be a closer match.  Maybe go with something a little heavier on the A as well, if it seems too weak to blend well with the G and D?