I could not find any documentation for a "Philips FC-C505".
I did find information about the Philips FW-C505
Although this would not be pro-audio related, and therefore off-topic on this SE-site, let me put in my 2 Cents...
PMPO stands for "Peak Music Power Output". This means absolutely nothing!!
It is a rating invented for Marketing purposes, and makes any speaker looks like it can handle a lot of power
My advice: IGNORE PMPO
The specification of the amplifier is clearly detailed on page-26 of the manual, and lists "2 x 60 Watts RMS" with a little footnote which details "6 Ohm, 1 KHz, 10% THD". This means the following:
The amplifier produces 60 Watts RMS when:
- with a speaker with 6 Ohm impedance
- at a constant tone of 1 KHz
- with 10% distortion
In regards connecting your own speakers to this unit. The specifications do not list a "range" of speaker impedances, on page 9 you find a small warning "not to connect any speaker with an impedance lower than 6 Ohms".
In that regards you are safe connecting your 300-Watts/8-Ohm speakers.
In regards to estimating what this would do for your power output, we can use Ohms law to calculate this. (Note: while the calculation is simple, the reality may be slightly different, based on the internal circuitry of the amplifier, which at this stage is unknown)
Lets start with calculating the Amps going through the default 6 Ohm speakers at 60 Watts:
I = SQRT(P/R) = SQRT(60/6) = 3.16 A
We can now calculate the Output voltage of the amplifier:
E = P/I = 60/3.16 = 19 V
With your new speakers the voltage stays the same, so we can calculate the Power:
P = (V*V)/R = (19*19)/8 = 45 Watts
Now this is only an "estimation" of power, but it should give you an idea.