Hmmmm.

I am confused by the signal chain in the thought process here.

You don't really "master" an MP3--you "master" the individual tracks that eventually get exported to a .Wav or other high resolution format and then you do one or 2 final "masters of the master", Ozone 6 for example has numerous tools for this.

IkMultimedia has this pricey (but totally worth it) plug in that has some great presets such as "Vocals out Front":

http://www.ikmultimedia.com/products/trmastereq432/

The point is, if you don't have many "mastering functions" done by the time you are ready to convert to an MP3 it is way too late, there is nothing you can do with a vocal that is improperly compressed, or EQ'ed or has the wrong effects once it is already in the mix.

MP3s can then by "mastered" by adding generic tonal shapes or boosts from any commercial products like Ozone (which you should have anyway) if you think your main outlet will be streaming.

But those MP3 masters for streaming services are not what you would generate if you want something to sound great on a decent home studio.

Any "mastering" done for soundcloud only works if your goal is to sound as loud and as innocuous and thumpy as 80 percent of everything on sound cloud.

****

Additional Notes on Mastering Plug Ins: I love this thing:


"The Master EQ432 module is based on the Sontec transformerless, solid‑state mastering equaliser. It uses switched EQ frequencies, with five switched Q values per band; the three parametric bands have very little interaction with each other. There's also a low‑cut filter switchable from 50 to 100 Hz, a high shelving filter and a gain control for the output. The user can set the maximum cut/boost range to 6dB, 9dB or 12dB. This equaliser is designed for fine‑tuning a mix rather than beating it into shape, and as such, sounds very subtle and polished. Adjusting the various bands brings about the required spectral rebalancing in a very musical way, with no loss of focus — which is exactly what you need when mastering. In a word, or two: clean and transparent."

David Snyder
Songwriter/Renaissance Man
Studio + Fingers

https://davidsnydersongs.com/
https://spoti.fi/359Gigs
www.davidsnydermusic.com/
www.reverbnation.com/davidpsnyder
www.soundcloud.com/davidsnyderchannel
https://www.songtradr.com/user/profile/david.snyder