It is not possible to have a positive 2-channel offset. If the 2-channel offset is set to -5db it will be 5db quieter than the PL2X setting. What you are saying sounds exactly opposite of this and does not make sense to me. The offset should go all the way to -8dB on the 990 and -6dB on the 970.

Maybe someone else can chime in and tell us were both wrong