In regards to isolation for headphones, I think there are a lot of bad assumptions/beliefs and/or misunderstandings about what’s possible.
For one, passive headphones will always be better at keeping sound in, than in shutting it out. For another, the amount of effective possible attenuation of external sound for over-the-ear designs tops out at about 10 dB. And that number is for hearing protection devices, which are specifically designed to shut out noise. Headphones will invariably be lower.
You can read more about it here. It’s relevant to all headphones, not just the Stellia. And it rapidly becomes a case of “right tool for the job” if you want it to be effective.
In regards to how much sound headphones keep-in, and the Stellia in particular, they’re a bit better in this regard than the Elegia, but, in the most relevant region (1 kHz to about 6 kHz) are as much as 12 dB “leakier” than, say, the non-vented (as far as I’m aware) AEON Flow Closed:
I’m a bit surprised that in an environment as noisy as any bus I’ve ever been on (typically about 90 dB in motion) that anyone can hear any sound coming from the Stellia unless they’re being run at levels I’d consider unwise (if not actually unsafe) and/or they’re sitting inappropriately close. If they are, they are, but I’m still surprised by that.
As to using an additional amplifier with some level of bass-control, I find it hard to imagine that the on/off nature of such things is going to justify the cost …
And the Calyx M does not, as I recall, have a proper line-out - which means you’d be amping its amplifier output. This is less than ideal. You’re probably better served with a new DAP around the same price as the amplifiers you’re looking at … $7-800 today would get you a DAP that beats out the Calyx M in terms of sound quality and features AND that includes a proper, high-quality, EQ capability (with both parametric and graphic configuration options), lower noise and more powerful output.
At the end of the day, its down to what you’re happy with. For me, that means “right tool for the job” which pretty much precludes using any passive headphone in anything but the quietest of public settings.