Pioneer SX-424 QUestions!

[ToyPick]

A friend of mine gave me an old PIONEER SX-424 amp.

The amp works fine, but the SPEAKER connectors were modified on the rear to get standard connectors.

Now what is confusing me, is that it is a 2 channel amp being that there is a speaker selector showing : Speakers A, Speakers B and Speakers A + B.

Here's what confuses me, on the back of the amp there are only 4 wires, blue, yellow, purple and green. These wires connect one PAIR of speakers and it seems that the modified connector has this pair set in "parallel" with the other set. So you can plug in 4 speakers, but it seems as though they would all be on the same channel anyway ????

What gets a little more confusing is that I can't seem to get any logical Stereo arrangement, there seems to be no LEFT or no RIGHT?

One last thing that's weird is the amp plays a **** load better when you unplug ONE of the input L or R in the sense that you have the amp playing with L and R plugged in at AUX. input and you remove while playing, one of the two and the amp's sound improves by like 40% easy the bass and treble get a whole lot clearer and cleaner.

Any thoughts on what's wrong with the amp? Is it good for junk?