Hi Krisgib
Do bear in mind that the speakers you own are 6 ohm rated at 40-120w. the amp is rated at about 50 when in 6ohms.
Surely even in regular one cable wiring the speakers are being underpowered, given that your lowest level of power to get these speakers driving is about 40w, and that the maximum power output of the amp is about 50w at 6ohms, it means that by the time you start gassing the amplifier - you are still only just starting to get the speakers moving.
Underpowering speakers is much more dangerous to the speakers than over powering them - primarily because when you reach the amps highest output you start to square wave the signal. This causes distortion in the tweeter at frequencies you can't hear - and its known that when you start feeding a square wave rather than a nice sinewave - the tweeters burn out.
The square wave distortion will not start occuring on a more powerful amp until the amp is near its maximum power output.
The main drivers are fine - because there's not anywhere near enough power to get them distorting at frequencies that you can hear. Thats why its better to over match speakers. At least if you start to put too much power to them - you'll find that the main drivers will distort and buzz - your instinct will tell you turn it down rather than continue.
Your Marantz is cutting out because you have not matched the speakers.
if you are using speaker A&B for what you think is bi-wiring, then you have not paid dues to the warning on the back of the amp relating to the impedances printed on the amp beside A+B. Which is 8-16 ohms. Marantz specifically have not mentioned biwiring in this manual - but did mention it in the manuals above this (8003, ki pearl lite etc). I had a 6003 but upgraded to the 8003 I bought second hand specifically because I was experiencing issues powering my floorstanders. Now I habve no issues as the amp is matched closer to the speakers.
You work out your total impedance placed on the amp by this formula (AxB)/(A+B).
In your case because you only have one pair of speakers there is no "B", so your figures are A:6ohm, B:0ohms
((6x0)=6 / (6+0)=6) = 1ohm This means you are placed excessive current draw on the amp.
As our esteemed mr everard (and much more experienced than I) states - just run from one pair of terminals only, modify if you have to - but run from one pair of terminals only
thanks