To clarify further:
1. There are two inputs on the back of the amp--RCA, and one XLR.
2. Both of them are tied together.
3. There's a switch to short the inverting inputs to ground so you can use the RCA inputs. It does not select between inputs, it only shorts to ground.
4. While it's possible to have two inputs connected, it's not a good idea. If the source equipment is on, you'll be trying to drive the output of one source to another. If the alternate source is off, you have no idea what the state of the output is. It might be disconnected via a relay, or you could have a relatively low-impedance path to ground. Hence, "choose one."
As to why we chose to do it this way--simplicity. This is an amp, not a source selector. The balanced/se switch is on the back of the amp.
If we went on adding a ton of features, like switchable gain, or a real input selector, or single-ended outputs, we'd end up with a more complex, sonically compromised, more expensive product. That isn't what Mjolnir is about. It's about maximum balanced performance at minimum price. Period.