You need to use a limiter (LMMS has a nice built in one called Fast Lookahead Limiter) for the crackling/scratching. Put it in the problem instrument's FX chain, and that should solve individual instruments. Put in the the master channel if you want to limit the whole piece.
For the "many instruments playing at the same time" thing, know about dynamic balance. Know which sounds should go above the others, and automate them. For example, in symphonic classical pieces, a when a crescendo happens, the bass gets louder, but the high, melodic lines quieten a little to be supported by the bass, and so this feels like it really is getting awesome. Another thing is, start off the song pretty quiet so you can build crescendos afterwards. It's just what I think, it might not work for you.
The song is okay, but slightly uninspired. A little (or a lot :p) wet reverb would've filled the ears and made this sound awesome, but by itself, it's just not bad. Rhythm is cool with the crash fills, but repetitive. Not much to say, but I do like some of your other stuff better.
And for the audience thing, I dunno. You've kept me here for a bit helping you out (hey, no inconvenience on my side, and I'm helping somebody out), but others? It takes a while to be known anywhere on the internet, but since NG doesn't show names next to songs, there's no guarantee anyone will want to exclusively listen to your music. The trick is patience, and self improvement.
By "uninspired", do you mean "repetetive"? I couldn't think of anywhere else to go with this, so I just repeated the same theme. Also, I just realized that this was my earlier version. I had changed the cymbals (making the brush as loud as the crash and making it less repetitive), but the changes are so unimportant that I'm leaving this one up. Thanks for the advice, though. I'll check out those effects.