I found something similar based purely on power level recently -- I have a Model 750 (165W per channel) driving mains, center, and surrounds and a bridged two-channel amp (100W per channel, 200W when bridged to mono) driving the single surround back speaker. The surrounds and surround back are identical speakers (3 Paradigm Studio/ADP's), but I had to crank down on the surround back channel by something like 6 or 8 dB to get the speakers balanced. I suspect you're encountering something similar with your mains on the Tandberg and the rest on the 1050.

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