Many thanks "Panomaniac" for the graphs. I am yet to see a thread on 210s with real graphs. This will be very useful for ref. sake.

From the impedance graphs it seems the dual 515s (assuming they are in the parallel mode) are really 20-24 ohms each (@ 1 K Hz.), as specified. It is surprising that the DC values are at 9 ohms, as measured by 'AA'. Or may be he also had measured the value in the parallel mode, and while the actual value was at 8 ohms (which also is confirmed by the posted graphs), the extra 1 ohm was the meter leads error.