Hi Steve,
I do not use any headphone amps and connect them with own-made Y cables straight on the RM32 aux/mix outputs, so then you nee 2 XLR (female ends) to one female stereo jack Y cable per headphone, and I can confirm that this works great.
Have not tested this with any other headphone brands besides Audio Technica (ATH-M50) and AKG. So not sure if a higher impedance Sennheiser go well with this.

Plenty of headroom, great full spectrum sound (perhaps just a tiny fraction less sub-bass sound but who cares, had no complains about bass yet), no broken outputs, all going well with minimal hardware and driving a total of 9 headphones simultaneously this way (including the connector on the RM itself). And no hassle setting levels at remote headphone amps. I know exactly what is going to the phones.

Perhaps some of us disagree because it is technically not the most elegant solution but it works for me for the occasional recording sessions where there is not a headphone mixers and I find it easy to connect. I can even use the many XLR looms that we have to hook everybody up if the distances in a bigger room require this. This could also prevent to invest in many jack extension cables.