....I am comparing my tracks mainly not to professional music, but tracks on Soundcloud that are similar-ish. 

That is one of your major issues right there. Most of the stuff on Soundcloud is amateur to say the least and the mix and mastering is far from good. You need to stop doing that!  There are way too many variables on people's music posted up on Soundcloud. Poor mixing and mastering being the main one.

You need to start listening to well mixed and mastered material in the same genre and not only that but you need to do it on your speakers in your monitoring environment. That way you will get to hear what great music sounds like on your system.

And when you are mixing and mastering you also have these quality reference tracks available at any time to be able to immediately switch to and compare.

Bottom end can be an issue but like normal music too your bass or bottom end needs to be sorted out as early as the synth sound itself, the track EQ etc and any HPF filtering that may be required and lastly just getting the right amount of bass in your mixes. Many people go wrong here and have far too much. Simple as that.  After getting what you think is the right amount of bottom end or bass in your mix try pulling down all the bass faders and then listen to your mix for 10 minutes without any bass parts present. Then after a while slowly bring the bass parts back in. You will often find the faders will be at a lower level than before once you get the right amount of bass bass back in your mix.

The small mono speaker down at low volume will really let you know when the bass parts are right. You need the right amount of mid range and high end material in your bass sounds and a little less of the deep subby stuff more than likely.

It sounds to me like you're comparing a .wav file in you studio to an .mp3 file on your iPhone.  For your experiment, you might find out what how much compression is being applied by iTunes.  Then you could get an mp3 codec, use the same settings and see if you get similar results in your studio.  Once you've done that, you'll know if it's something you have control over or if it's Apple's fault. 

iTunes has got nothing to do with it. Like I said in my original post a great mix and mastered track will transcend all the mediums it ends up on mostly. I have done some great mixes and mastered tracks in my day and many have ended up on iTunes as well and the tracks ended up sounding great there. iTunes did not change a thing. 

Ultra,

-12 dB or so should be fine for recording.  Rather than using a compressor/limiter to make it louder, use the faders.  Compressors/limiters should be used for evening out the sound, compressing the louder parts so they are softer and seeming to raise up the level of the entire track.  Eq is best used trimming down the frequencies you don't want to hear.  The easiest thing to do is treat the mix like a subtractive synth, removing what you don't need or steps on another sound.

But it sounds like the small, reflective room is the problem.  It is very hard to identify problems if there are a lot of them.  A good monitoring system in a good room is easy to learn the problems - w/ enough time.  A bad room can make your mix sound like swiss cheese on other systems.  That may also explain why you have to push your mixes so hard.  Whole bass notes and even wider mid and upper range frequencies can disappear or get reinforced in a wobbly room.  You'll mix to it, and elsewhere those problems pop out.  iTunes, mp3s etc. can exacerbate them, too.

W/o moving to a new, treated studio room your best bet is to use some good headphones to check your mixes.  I wouldn't use them to mix, but rather use them to spot check your songs.  The mix should be getting close to -6 db or so.  Then don't limit them so hard.  I'm not familiar w/ ozone but optical comps are slower and will let more of the initial hit through.  Don't worry about other songs on soundclick etc. or the radio (radio stations have their own compressors, etc.).  The idea is to get the best mix, even if it is a few dBs down.

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