There is a movement speed bonus cap. At 60, I believe there is no difference between using Selos or a Beast drum.
The difference is that your Selos increases each level. So a 55 Bard, with DoTB, might run as fast with a 60 Bard with Selos. I'm not sure. But theoretically you can cap your movement speed pre-60, I just don't know what level that is.
This spreadsheet allows you to put in instrument modifier and level, and it spits out the Dot, Reist, etc. damage
https://docs.google.com/spreadsheets...hl=en_US#gid=0) allows you to put in instrument