I'm the odd man on this one, but I personally master for the song and have never paid attention to iTunes or any other online site. I've never noticed any issues with my masters for people other than when any encoding below 192 is used for mp3's. Anything 192 and above, and I hear no differences that make me wish I had taken another approach.
In my personal opinion, I do not believe the actual dB reading is the issue. I can squash the heck out of something and keep a sane level....and that is where you will hear changes when something is encoded. If you don't kill the compression and literally master for the song as far as loudness goes without killing your limiter threshold, none of the final output volumes have made a difference negatively.
For example, it is rare for me to go louder than -4.5 to -5 on my limiter threshold and I like to keep things at -0.3 to -1 depending on what may sound better for the song. Years ago it was nothing for me to slam -6.8 to -7 on the limiter threshold. The more you comp the mix and squash the life out of it, the more encoding will kill it no matter who does the encoding. For that reason, I honestly never pay attention to final dB numbers unless I specifically have to for a TV or film client etc...or someone that asks for something specific.
My advice? Let the music breathe...stop comparing to some of the commercially mixed garbage that caters to loud, lo-def music that only sounds good on earbuds....and you set the example while making people follow you for great fidelity, dynamics and for being yourself. I have a nice little gang of dudes that literally hold some of my mixes as reference mixes. Some of them are higher in the food chain than myself. It's the ultimate pat on the back to have people bench marking their material off of mixes you may have created.
Whether I suck as an engineer or am doing things completely wrong or not....I got some people I hold in high regard that think I'm onto something. Don't be afraid to be dynamic and turn up the volume knob. As soon as we remove hyper-compression/limiting from our worlds, all the stupid things I see being debated and discussed on message forums will go away and people will concentrate more on just learning to record with quality, and releasing product faster.
Seriously....do you know how many threads get posted on ALL forums about how limiters and final volume outputs are important to people? It shouldn't be important because if you think about it, it's the biggest load of artist stifled creativity ever brought into the marketing world. Seriously think about all the volume related things you have read and how many times you or others you know sat with a mix and tried to get it as loud as something commercial.
Then think about how much time was wasted in doing that. No one other than you and any engineers that think they are special for contributing to the loudness/limiter wars will care. Music lovers just want your music. They could give a rats @ss about LUFS, the K system, dB's or how much time was spent loading and re-uploading to iTunes hoping for that magical mix. Keep your volumes sane, your limiting to a minimum or slightly aggressive when need be....and never worry about any of this stuff again. Trust me on this one. :)
-Danny