What are "true" balanced connectors

The Odyssey Stratos amp is not a true balanced amp.  The inputs are there strictly for convenience and if you look inside, you will see they are connected to the RCA jacks.

Balanced inputs are not defined by whether two channels of a stereo amplifier are bridged or not. The concept of balanced inputs is that the input is a differential receiver, where the signals on each wire are equal and opposite polarity. Thr differential receiver subtracts these two signals to reproduce the original signal. The benefit over single ended signalling is noise rejection. If the cable is exposed to a source of electrical noise, that noise will be induced with the same polarity and amplitude on both signal bearing conductors, and since the differential receiver subtracts the signal on both conductors, it cancels out the noise, which is common on both sides of the differential pair. This is how the term “common mode noise rejection” was coined. 

sleepwalker is correct.
A "true balanced input" means the + and minus of the input are differential, and not referenced to ground.

A "true balanced" amp means that both outputs are driven. Most amps only drive the red output.

The Parasound A21 as well as several of the Yamaha P2x00 amps from way back are examples of amps with XLR inputs, but they are not balanced.

Each signal is actually referrenced to ground, but the differential receiver only uses the two signal bearing wires to convert the differential signal back to single ended for further handling. 

Each signal is actually referrenced to ground,

Not universally true.

Best,
E