The "750" was based on peak at 2 ohms, but it was actually a 150x2 RMS amp at 4 ohms

Even RMS by itself is a bit meaningless. You still need to know what distorsion is produced at that power and what frequency range they used. This may be covered in another thread but things like THD (Total Harmonic Distorsion) should be included in the rating.
Your 400W Max mega sound system may mean that the speackers actually started burning at 400W but stoped playing anything meaningful at 120W and had 50% distorsion at 50W. They were also playing a 100 Hz Sine wave through it at the time.

If you had a speaker rated as 120W RMS, 0.06%THD @ 180-19KHz, then you have a more meaningful description of the speaker / amp. Get this kind of reading on a 'el cheapo' speaker? I think not

Murray 06000047
I don't think, therefore I am not.