Hello Mort,
An RCA to XLR cable (adapter) will only be using a single "leg"(signal) of the balanced XLR cable. The XLR cable is intended to carry 2 "legs"(signals), each out of phase from each other. The use of 2 out of phase audio signals allows the noise picked up along the cable to be removed(rejected) at the end, inside the amplifier. This is called CMR (common mode rejection) and is the primary reason XLR/balanced connections are used.
If you used the RCA to XLR cable it would produce sound but you would be throwing away any advantage of a balanced cable. The BobWire XLR1 will take the RCA signal and convert it to balanced, using active phase inverters. This means the output of the XLR1 will be fully balanced and you can enjoy the advantage this brings as the signal travels to you amplifier.
The 2nd reason is just as drodgers suggests, you will not get the 6dB of gain that the balanced XLR connection provides. This means, if you are using an RCA to XLR cable, the audio level would drop 6dB when you try and listen to the source with othe RCA outputs.
As you mention, Amir found the SINAD was higher using the RCA input. This would be expected because the RCA input is going through the phase inverts to produce the true balanced signal. What you perhaps are not taking into account, is that if you use RCA to XLR cable, your amp is only "seeing" half the balanced signal. This totally removes the advantages of the balanced inputs on your amp. You will also need to turn your preamp/source up more to compensate for the 6dB lower signal(adding to the SINAD). So there is a give an take here but I believe the end result will be better using the phase inverters of the XLR1 rather than a RCA to XLR cable.
-Bob
