Amplifier – Where to Hook Up a Speaker in a Schematic

You have asked a lot of questions. I'll try to get all of them.

Your power calculation is correct. You will see about 1.77 Amps RMS current whey your amplifier is delivering its' rated power of 25 Watts into an 8 Ohm load.

1) The reason your measured power is so much lower is that most of the time, the amplifier is delivering only a fraction of its' rated power. In addition, your multimeter isn't responding fast enough to show the peaks. An oscilloscope monitoring voltage across a series shunt resistor will give you a better idea of the peak current.

2) Your speaker volume controls are most likely "L-Pads". This is a wire-wound potentiometer with two resistive elements that change simultaneously. They are configured to keep the impedance as seen by the input terminals (the amplifier) reasonably constant as the attenuation is changed. Wikipedia has a good description.

3) Speaker efficiency varies greatly. Some speakers can deliver significant volume levels with tiny amounts of input power, some speakers require large amounts of input power to achieve even modest volume levels. It all depends on the complete speaker design: drivers, cabinet, crossovers (if any).

It is not likely that there are hidden power amplifiers driving your speakers but is it possible.

It might be slightly less bad but the ratio between maximum excursions is only 1.6, giving you 4dB more (potential) volume - barely perceptible.

You'd be better off either moving to a larger radiating area - a bigger driver (a 4 inch woofer is an oxymoron!) - look for one whose output doesn't roll off before the bottom of the band of interest.

Or if you can't afford the space, perhaps a tuned port to augment the output around 100Hz. Port tuning can be tricky, but there are loudspeaker enclosure design programs that will help. You're probably looking at an esclosure volume of about 4 litres with a fairly small diameter (25 or 30mm) port as a starting point.

Alternatively : is the speaker being driven outside its linear range by frequencies below the band of interest? Perhaps a high pass filter set to 80Hz will pass all frequencies of interest but protect the speaker from high amplitude 20-50Hz signals that can only drive the cone out of its linear range and potentially cause damage.