First, understand that speakers are not a pure resistive load, so "watts" from the amplified doesn't equate directly to "watts" at the speaker. Most amps give rated power into strictly resistive load (e.g. 8 ohms, 4 ohms, etc.) and if the speaker was a resistor, then yes "watts is watts". However, since the speaker receives an A/C signal, and there are various elements in the speaker that have capacitance or inductance, you need to think in terms of "impedance" not resistance. Meaning, the signal voltage and signal current at the speaker will not be in-phase (i.e. max or min current won't align with max or min voltage), and that difference is called the phase angle. For a real world speaker, the phase angle can reach +/- 45 degrees (for a resistor, that angle is zero). So the actual power delivered to the speaker is a function of the cosine of the phase angle, and this is basically the Power Factor, and the power supplied by the amp. So this can be as low as 0.707 for a real world speaker, meaning the amp must supply ~4 watts at the output devices for the speaker to "see" 1 watt. Worse, the phase angle will change for different frequencies, so the frequency response (not just overall volume) is also affected when the amp can't supply sufficient power. So you have to figure that a 50w @ohm amp/receiver is likely to be capable of less than 12 watts for a given "4-ohm" speaker at worse-case (relative to power factor) frequencies.
Better quality amp manufacturers will take phase angle into consideration and have a sufficient number of output devices and power supply stiffness to accommodate the increase in instantaneous current load requirements (headroom available) for most real-world speakers.
Can't comment on the Maggies specifically since I'm not a dipole kind of guy.
Good luck!