``I found the Rx MK3-B was absolutely quiet enough to use with high-sensitivity in-ear monitors...... the ALO produced no discernible hiss or hum when used with either my Westone Elite Series ES5 custom-fit in-ear monitors or my Ultimate Ears Personal Reference Monitor``.
The MK3 is great with full size headphones, but with in ear monitors like ( JH5 PRO, JH11 PRO, JH13 PRO, UE Triple.fi) for example I have some complains. For me there is a great ammount of channel imbalance at low volumes, and the sound kind of warbles in and out with some soft static when turning the volume knob slowly between the lowest volume and a normal listening level =/ I did not have this problem with my RX MK1... I bought the MK3 to use it with my customs IEM and I was disappointed because of that. I bought a balanced cable to use with my customs and it gaved me more room with the volume nob, but still not how I wanted, so I sold my MK3-B and I`m thinking of buying the Alo MK2 or RSA The Shadow. I hope Alo Audio improve it for the next model. The guys from Alo were very nice and offered me to make an adjustment, lowering the gain settings and making my IEM experience improve measurably, but I`m not with the amp anymore and it would cost a lot of money to send it to the USA and pay customs to get it back here in Brazil.
So far as I am aware, you may have encountered a problem that (I'm told) arose with a handful of early production Rx MK3-Bs, and that has since been resolved by the very gain adjustment you mentioned. Once that adjustment has been made (and my understanding is that it is now a standard feature of production Rx MK3-Bs), the unit should be very quiet when used in balanced or single-ended mode, and with IEMs, 'phones of average sensitivity, or hard-to-drive 'phones.
'Sorry to hear you had difficulty with your sample of the Rx MK3-B, but it does sound like you have access to some very good alternatives. Note: If you want a balanced amp that is, arguably, a bit quieter than the ALO, do consider RSA's SR-71B The Blackbird.
Best, Chris Martens
Editor, Avguide.com/Playback/The Perfect Vision