I have used an Asgard 3 with good success with the Bifrost 2 but recently I moved the A3 to another source chain and I am now using a Jotunheim 2 with the Bifrost and also a Monoprice Liquid Platinum. Both amps are using the balanced out from the Bifrost 2 via an XLR switch. Both of these amps sound terrific and are a good match for the Bifrost 2.
If you are looking for something else, perhaps a Lake People G111? I have not yet heard this amp personally but @WaveTheory just recently did a review of the G111 and discusses some about which DACs he used with it - Bifrost 2 being one of them. You can find that review HERE. I’m sure he could also answer more questions you may have about amp pairings with the Bifrost 2.
Edit: I missed the part about iems. The Liquid Platinum is not a particularly good candidate for iems as the gain is pretty high on that amp, and the volume pot is known to be pretty sensitive.