Since most portable players do NOT have great sound systems, one needs to produce a good sounding audio file. The choices seem to be limited, in the main, to mp3, ogg and wma. The latter two at 96 bit are about the same in sound quality as mp3 at 128 bit, so you save space by using either ogg or wma when your player will work with one of those choices.
To convert a midi file [mid, midi or kar] to an mp3, ogg or wma file, MY preference is to use a program called SynthFont, and use higher quality sound fonts. Search for a Steinway B [Perfect 2 works well] sound font and RealFont version 2.3 and you will have covered virtually all your needs... Simply point SynthFont to the directory of midi files and have it convert all of them into your choice of WAV, OGG, APE, MP3, FLAC or WMA music.
I also use a feature in SynthFont with piano music called PAN spread. I use 200% spread, as if I were sitting in front of a piano's keyboard. Try it and compare the output to see if you like, or dislike, PAN spread. Some users like it, while others hate it.
I also use MidiNotate Composer to change, or correct, midi file setup when I do not like something, such as the BPM, or the instruments chosen, or if I want to merge or split staves or hands differently. The current program is Notation Composer, and it happens to be on sale right now at a reduced price, but I have no idea how long that 'sale' will last. I use it so rarely that I cannot justify the acquisition.
Anyway, SynthFont is 'donation' ware, and is out of Europe, while I am in Canada and simply provide suggestions to its author and maintainer, Kenneth Rundt. Try it and if you do not like it, simply delete it from your computer. As for where it is, just do a search for SynthFont in Google or another search engine and you should find it easily.