Kenwood vr606 reciever shutting off subwoofer and centre speaker simultaneously after few minutes/seconds of powering on. WHat's the cause?

check and test the circuit for damage short or overheating

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Several possible answers. First - is that the Yamaha has a fault that is sending a signal with too much power on it. And the sub detects this and switches off.
Second - either the Receiver or Sub is switching off when no audio is present. A setting problem?
Third - the sub has a fault, possible overload, and goes to standby for safety.

hdmi is a direct connect it usally wrks

new reciever

There are 2 ways. The first is in series. Connect in this order (amp)+ to (speaker1)+, (speaker1)- to (speaker2)+, (speaker2)- to (amp)-. Repeat for second channel. The amp will run smoother but you cut your power in half. The second way is parallel. (amp)+ to (speaker1)+ to (speaker2)+, (amp)- to (speaker1)- to (speaker2)-. Repeat for second channel. This will double the power but will cause more heat and distortion. Too much heat and distortion make it no worky.

does anyone know where i can get a kenwood cv 751 from or parts for this unit?

try removing all the speaker cables and power back on, a loopback causes this same effect.

POSSIBLY A BREAK IN THE SPEAKER SUPPLY LEADS TO THE ACTUAL BASE UNIT
WE HAD THIS AND IT WAS NOT THE SPEAKER BUT THE BOLT ON CONNECTION UNDER THE SPADE TERMINAL ON THE BACK OF THE BASS UNIT

I second this problem, I'm having the same one. Hey, Bosun, did you ever figure out anything?