Around 4 seconds that bass roll seems a bit unnatural, maybe do an arpeggio instead? It's much easier to do a quick succession with an arpeggio on piano then it is to hit a single note multiple times
The piano is a little thin but it doesn't sound too bad (it's lacking in the mid and low end, but that's pretty normal for free pianos as far as I've seen), when the other instruments start coming in though.....
Yeah you should really look into some sound improvements.
I don't know what you're using but from what I hear (I don't use soundfonts myself) these are pretty good orchestral sounds:
http://the-filmmusic-group.deviantart.c ... -224143947-When the Flute comes in it's really, really weak and hard to hear - also it's articulation sounds rather unnatural
-Honestly I'd suggest changing the electric guitar to something else, fake electric guitar never tends to sound very good.
-The string sounds aren't too bad really, they're not amazing but they are workable - same with the brass.
-The drums could use a lot of work. If you need some better Drum and Bass samples I could send you some, but most drum samples come in Wav format - from what I've seen most soundfont drums are very weak in general, if you have a drum sampler like Battery or something it's much easier to work directly with the audio files, I tend to just work things with the audio directly using audio tracks when it comes to drums. The drums just feel almost nonexistant aside from the snare through most of the song, they should be a bit more in your face, a bit more driving - for the jazzy effect you don't need them like RIGHT in your face like DnB drums normally would (I've noticed a lot of DnB and Breakbeat influence in the drums, it's really neat and goes pretty well with the sound) but some more presence to drive the song could definitely help.
But man the composition on this is done really well, all the attention to detail is done really well (as a bassist someone who pays attention to writing a good bass line is me gusta, so much). You've definitely hit the mark on jazziness. Really the only complaint I have on the piece is the mixing, which honestly a lot of it isn't your fault if this is mainly free resources, since they tend to be a bit thin in general.
As for Reverb:
Effect loops man, they work wonders.
Basically in whatever your music program is you set up an Auxiliary track, put EQ on it and EQ out the low ends and then put your reverb plugin with the mix at 100% wet after that.
Now from here take the tracks that need reverb (generally the higher something is, the more reverb it'll need - most of the time the bass or the kick drum have very, very little reverb since low frequencies muddy up the sound) and using FX sends bus them to the auxiliary track.
So what does this do?
Well for one it has your whole track running on one Reverb plugin, thus saving processing (really nice if you're using some fancy reverb like convulsion reverb which tends to use more CPU).
But the real benefit is that because the reverb is an FX send, the clean sound of the un-reverbed instrument is still coming through as it would before with the wet reverb sound playing behind it. This way it doesn't sacrifice clarity like using reverb normally does (the more you turn up the mix, the less clear it becomes if you just put the reverb directly on the instrument track - because it's on a separate channel entirely we don't need the clean signal coming through it, hence the 100% mix).