When I presented the problem to Arriva, the company suggested I try Acoustibuds, silicon tips that slip over the top of hard earbuds and essentially turn them into soft earbuds. In my case, the Acoustibuds made a world of difference. The headphones became much more comfortable and I got a better seal, which had a big impact on improving bass performance.
About that sound: it's not great, but it's decent enough. Once I got a tight seal with the Acoustibuds, I was pretty happy with the sound, though these guys just don't offer a lot in the way of clarity. Some of that's simply because of the quality of the drivers and some of it's because of Bluetooth streaming, which compresses your audio. In any case, the Leo set is designed for gym and outdoor activity, where a secure fit rather than great sound is a higher priority.
Starting in February, Arriva says it will ship a pair of medium-size Acoustibuds with the product and it will also offer Acoustibuds for sale on its site. But in the meantime it says it can offer the medium Acoustibuds to anyone who would like them ("free to all those who have ordered a Leo to date").
I mention all this because while I like the design of the Leo, it only becomes excellent for me with the Acoustibuds (as I said, others may get a perfectly good fit with the included silicon covers). So it's good to see that Arriva's willing to make good on its promise to have the Leo fit all ears well, not just most.