I agree with this... when combined with the Sennheiser HD650s (balanced), I find the ALO Rx Mk3-B+ to sound a little distant, with mediocre soundstaging and imaging, being overall a generally boring sound-signature (with or without the bass-gain knob turned on.) However, when listening to the HD650s out of my Duet, the soundstage and imaging become quite impressive and expansive, making the overall sound-signature of the HD650+Duet pair quite fun to listen to.
With lower impedance IEMs like the Sennheiser IE800, the Rx Mk3-B+ does have a noticable background hiss, but I found the sound-signature to be a little more enjoyable than the Duet, although the Duet beats the Rx Mk3-B+ in terms of detail retrieval and imaging to a slight extent, probably stemming from the Rx Mk3-B+'s high noise-floor with lower impedance, high-sensitivity IEMs. Generally, however, I've been using my Duet a lot more than the Rx Mk3-B+ these days, both with my IEMs (the lack of background hiss/raised noise-floor is nice) and higher impedance headphones (e.g., Sennheiser HD650.)