Ah yes, watt in speakers.
First of, it can mean two things. That is the power you put in it, or the power it generated.
Generally they mean the same, but it gets confusing if you switch from a single speaker to mulitple speakers. And it sounds like you're going from stereo to 5.1?
Then there is what it means in loudness.
So watt is a linear metric.
Loudness, is usually measured in dB which is a logarithmic metric.
Meaning that your speakers of nearly double power will give you . . . . . 3dB? extra [strike]loudness[/strike] sound intensite. This is measurable and noticable. But it generally requires 10 dB to double the perceived loudness.
So 75 will mean more power drawn and a bit more loudness.