Incorrect.
A 25W amp that's fully biased in 8 Ohm does 18.75W continuous Class A power in 6 Ohm.
A 20W/8 amp does more than 20W continuous in 6 Ohm, theoretically ~26.5W, but due to rising distortion level something like 24W for real.
20W Class A in 4 Ohm enables it to deliver that 24W in Class A, theoretically 30W if rails would allow.
Obvious he is not hearing a difference in power, ( the small increase is trivial if any ) for the amplifier to open up in the way he has described is interesting , is it due to more class A bias or the fact that by reducing the voltage, he has hit on some kind of sweet spot? what is really going on here ?
yes i did.I used RCRC filter.It reduced the voltage.It is known that reducing voltage and increasing bias is better for lower impedance speakers.I asked Nelson about Krell ksa50 to drive low impedance speaker.He said that "lower the voltage as you wish and increase the bias.It works perfectly" Maybe it is not same to F5.I do not know.Let us ask Nelson.He knows better.Who will ask to teacher ? 
Hiraga used CRC "filter"
And also a "bipolar cap coupling", by having two supply caps in series, between rails and ground
Not very cost effective, as you get half cap value, but improves voltage rating, thus can use lower voltage caps with very high capacity value, I suppose
Someone once told me that classA amps like power supply with some resistance
I dont know, maybe you get less voltage variation, less "pumping", and draws more current
Even if its very small amounts
But its not very costeffective to lower voltage that way
Considering that a lower voltage trafo has better current rating, with same wattage
Im sure what is suggested in F5 manual is optimal
But hey, components are different, amps are different
Jacco, please correct me, your answers are good
And also a "bipolar cap coupling", by having two supply caps in series, between rails and ground
Not very cost effective, as you get half cap value, but improves voltage rating, thus can use lower voltage caps with very high capacity value, I suppose
Someone once told me that classA amps like power supply with some resistance
I dont know, maybe you get less voltage variation, less "pumping", and draws more current
Even if its very small amounts
But its not very costeffective to lower voltage that way
Considering that a lower voltage trafo has better current rating, with same wattage
Im sure what is suggested in F5 manual is optimal
But hey, components are different, amps are different
Jacco, please correct me, your answers are good
Its not that I dont agree, but why not, whats the problem with a resistor, apart from powerhandling
Its used a lot in small signal designs
Is there any way to make an electronic resistor
I think mr Thel from Germany have talked about an "electronic choke"
Current demand is not constant. it can vary from 0 to twice the bias. So a resistor is not welcome as it will make the rails sagging on current demand.
A serial regulator can be considered as an electronic resistor
More bias allows deeper class A with 4 ohms speakers.
Then the rails are lowered to stay within the SOA.
I wont exceed class a mode.I think there will be no problem then
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High bias is very desirable, but you have to have really good
heat sinks to insure reliability.
You can decrease the voltage a bit, but remember that the
capacitance of the devices is a function of the Vds, the voltage
from Drain to Source, and below 5 volts Vgs or so the
capacitance gets excessive. At +/-18V I think you can safely
run 2A bias, but you have to watch it, particularly because
the Fairchild parts are more sensitive to temperature vs
Vgs than other Mosfets.
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