SOZ 4 ohm load

SOZ and 4 Ohm speakers

I have a pair of variable power output SOZ mono blocks driving large three way 4 ohm 87 dB efficiency dynamic speakers. Ten watts is just barely enough. On big orchestral climaxes or other very demanding spikes in music with the volume turn up the sound would compress or in a worst case situation the midrange cone would start to break up a little. At 15 to 20 watts the problems went away. In fact you think your listening to much larger amps.
 
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Thank you all for your answers.

I appreciate Mr. Pass suggestion to half
the power resistors, this would imply
4 omhs for R1-R4 and 0.5 ohms for R5-R7.

I already acquired two 500 VA toroids with
20V secondaries, may be I get too high
voltage rails to match the suggested reduction
by 30%?

I would like to use anyway those toroids, is
any other adjustment that can be performed
maintaining the supply to about 25V for 10W
output?