Audio Systems: speaker distortion from analog inputs, 4 ohms, money reading

Question
My receiver has an A/B switch, and I have speakers hooked to both outputs. The speakers only sound clean when connected to the "A" output, and then the source must be connected through the digital inputs of the receiver (Onkyo TX-SR505). I've played through two sources, and both the problem occurs with both sources, and after reversing the speaker hookups. Both sets of speakers sound great when connected to the "A" output and the source is connected digitally.
The receiver is 7.1 channel, but I only have the front speakers (A) and the second room speakers (B)connected (I'm planning to purchase more speakers in a few months).
This is a new receiver, but the problem occurred with my former receiver, which had no digital inputs and distorted everything. I've never heard of this before and am at a loss.

Answer
I'm no consumer audio expert. I think anyone who specializes in it is a rare breed. There are hundreds if not thousands of receivers on the market. A single manufacturer has difficulty keeping up with their own troubleshooting let alone all of the configurations, menu options and idiocyncracies of each manufactured receiver. The old days you asked me a question about distortion and I/O connections and I'm right there with an answer that is usually on the money. Reading into your description I can only imagine that there are sub menus that allow you to control the input and/or output levels and that the distortion is a result of overload of one of the two signals. I strongly suggest reading the manual and if that doesn't help call Onkyo tech support. I think you will be hard pressed to find anyone outside of the Onkyo tech team that can make sense of what appears to be proprietaty DSP processing.

Be aware that shorting the outputs of any channel on the receiver or speaker can surely damage the amplifier. You should also check the impedance of the speaker and amplifier. Make sure your speaker is no less than 4 ohms and that you aren't using more than one 4 ohm speaker on a channel.