There is a reason for this. As UPMAN has said, the external speaker output in the 536HP is not like it was in the older models - it uses a BTL amplifier arrangement - BTL stands for "Bridge Tied Load". In essence, it means that what amounts to two mono amps are fed together with one having an inverted signal relative to the other. Think of the difference between -3 and +3 as being a total of 6 (the BTL; don't confuse this with "adding" the two "out-of-phase" signals and getting a cancellation, 0, as what we are doing is dealing with the "total difference" between the two signals and using that) versus the difference between 0 and +3 being just 3 (as in "normal" single ended grounded shield amps). That "difference" is what counts in the end at the speaker. So, essentially what you get is a very powerful audio signal for driving a passive speaker. The problem is, that is ALL it was designed to drive! It was NOT designed with any intention of being connected to a single ended device (with the shield grounded as with most consumer amplifiers, PC soundcard inputs, etc.).
So what happens when you connect a BTL amp to a single ended amp or other input device (one that has the shield grounded as in most consumer electronic gear)? Well, you end up grounding out or "shorting" one side of that BTL signal. Depending on the design of the BTL amp, this will cause, at minimum, low audio and/or distorted audio and at worst permanent damage to the amp.
So how do you connect a BTL amp output to a single ended input like a PC soundcard or external amplifier? WHATEVER YOU DO you MUST make sure that NEITHER the "tip" (center conductor of the speaker jack) or "ring" (outer ring of the speaker jack) are in any way directly connected to ground!
