Can DAP Drive Inefficient IEM?

I’m quite new to the hobby and am looking to upgrade my iems. I’ve been reading through the threads, but I’m not sure I can answer (or find the answer) my question.

I have the Astell & Kern SR15 that has output of 4.0Vrms balanced (condition no load). I have been trying to read up and figure out how much power this equates to, as I would like to buy some IEMs (and perhaps headphones) that are more difficult to drive.

How can I figure this out? If the headphones are, for example, the Sony Z1R at 40 ohms and 103 db/mw? Or another set that are 60ohms at 110 db? Will the SR15 power these IEMs?

Or perhaps I was thinking of the Focal Clear MG at 55 ohms and 104 db/mw?

Am I at the point of needing to add an amp (or desktop amp/dac combo) to get to the point of enjoying these IEMs/headphones, or can something like the SR15 be able to power them?

Thanks for any advice guys!