What I'm still trying to figure out is the best audio format for each site, or if Amazon, YT, iTunes are all best utilized with different file types. There's a lot of work going into it, so it'd be unfortunate if I weren't optimizing the sound on each site.
Like BSG said....I would focus on getting your best mix on the monitors you trust, and then just make sure your loudness numbers fall into the acceptable range for streaming. You can certainly go louder, and they will still take your submissions, but then their various algorithms will crush things down to their allowed loudness range. So if your mixes are already in that range, they will basically be untouched by the streaming service. If too low, no problem, they will boost...but too loud, and they will nock the levels down, so if you're stuff is pretty compressed, it will get mashed again.
If you were going to churn on physical CDs...then you can kinda do what you want with levels, and make them really louder, especially if you're going to do your own CD distro...but if you were then going to use the CD audio for file generation to give to streaming services, etc, then you're still faced with it being too loud off the CD for them, and again it will be reduced.
Actual on-air radio play has it's own crush method to keep things in acceptable broadcast levels...same thing with TV...etc...but IMO, if you stick to the acceptable streaming services levels...then your stuff should translate vey well to other distribution/broadcast services and formats. I know some people scoff at this, and say they want their stuff LOUD...but the reality is that too loud is going to get crushed by just about every streaming/broadcast provider...so for people who care about the sound quality, erring on the side of less loud actually works more in their favor than going on the too loud side.
If you're thinking about mixing things a little differently for this service and that service, and radio and this and that...I honestly wouldn't waste my time.
Just get the best mix you can on the system you know and trust...and live with the differences that will occur between streaming/broadcast services...just like you have to live with the fact that there are various playback systems that people will use, and different quality playback hardware...so that's nothing you can control, and you will go nuts trying to create multiple mixes to cover any/all playback possibilities.
The reality is that any non-lossless format will degrade your audio quality to some degree...so when you're hearing your mix in 24 or 32 bit splendor on your DAW and studio monitors, it just won't sound quite the same as a low-grade MP3...but that just is what it is. I wouldn't get hung up on that.
If you really want to know how all those differences might sound...take your mixes and churn out a few levels of MP3 files...burn a CD...go out to your car and to someone other playback systems, shuffle through the various audio files, and see how things measure up for you. I think if you have it pretty darn good in the studio, the rest will translate well enough on other systems and formats.
