OK guys, here is an answer from an EE perspective:
Like Maranketz wrote: all watts are created equal 'cuz it is a measure of power. Nothing hokey here. Just plain simple math: power = current squared * resistance.
Class A power amps have the least distortion thus the best sound quality 'cuz the output stage devices are biased so that they are always on. The maximum output current deliverable by the amp ALWAYS flows thru the output stage when the amp is powered on whether there is a load or not. If there is no load, the output current flows from the source transistor/tube to the sink transistor/tube into ground & is dissipated as heat. When there is a load then some/most/all (depending on volume level setting) goes to the load/speaker & less is dissipated as heat. That is why class-A amps run cooler when hooked up to a load/speaker! Theoretical max. efficiency for a class-A amp is 25%. "Elizabeth": your Forte amp would dissipate 200W while idling. That is not a lot, as you will agree, & I'm sure you'll also agree that 200W won't light up your entire house. It will be a diff. matter with some of Pass' class A amps like the XA-200, which would dissipate 800W while idling!
To answer "Tabu's" question: the answer is not clear cut 'cuz you have not told us which power amp can provide more current. It's all about how much current a power amp can dump into the speaker load. The 100W class-A can provide 3.53A max. of steady-state current. The 200W class-AB can provide 5A max. of steady state current. All this is well & good. The thing is that the music signal is NOT steady-state! It is a dynamically moving signal. So, if the power amp is to be true to the music then it must
* track the music signal - slew rate of amp
* provide sufficient current while slewing
The amt. of current providable is a function of the power supply & power supply capacitors. Why do think that many huge amps (take Pass for eg. weigh a ton?) It's that honking xformer & those massive power supply caps! large xformers are able to draw large amounts of AC current QUICKLY, which is converted to DC & used to charge up the power supply capacitors, which act as charge/current reservoirs. Any time there is transient (kick drum/bass riff, etc) the amps sends out a burst of current (taken from the caps) into the speaker. Only if sufficient current flows in the speaker voice coil can the cone driver create pistonic motion to create sound. Low current output from amp => weak cone driver motion => loss of sound.
A good eg. of a powerful class-A amp is the Clayton M-100. It puts out only 100W but it can supply 200 Amps of current! I believe that this amp can slew rapidly too using some hi-power Motorola BJTs. This amp has a lot of oomph & can drive a variety of speakers. Hell, one could weld w/ this power amp!
So, without further info, I'd be hard pressed to say which would be better. The 100W class-A might just be enough IF it has sufficient current drive. Listen to both, if you can, before deciding. From a pure distortion perspective (forgetting everything else), the class-A will always sound better since the output devices are always on & there is hardly any xover distortion compared to a class-AB amp. There are several class-AB amps today that are heavily biased towards class-A so they *almost* have a class-A sound. These are, of course, better than class-AB amps weakly biased towards class-A. Pass' X series amps are class-A for the 1st 30% & then class-AB. However, if you buy a X250, for eg, you will get the 1st 75W in class-A, which is all you really need for 90-95dB SPL listening levels. It is also one reason the amp is expensive - you are *practically* getting a class-A amp. The heat sinks on the X250 also tell that story.
"Elizabeth" if I had to guess why your elect. bill is higher w/ the Infinity I would guess that the Infinity's xover is more complex & power hungry than the Maggie's. I know that if the speaker designer doesn't use the "correct" Zobel network on the woofer drivers, the woofers present a varying impedance over freq. i.e. they are not a constant 4 Ohms or 6 Ohms or 8 Ohms, etc. This means that the amp has to source current over a varying impedance. Using the equation: power = current squared * resistance, you can see if resistance varies, power varies in direct proportion. This is, of course, DC power. But where does the DC power come from? From the AC outlet, which is AC-DC converted & stored in the capacitors. If the capacitors are drained quicker when the woofers drop to a low impedance/resistance, larger AC current has to be drawn from the outlet to replenish the capacitor charge. This will affect your elect. bill. In comparison, the xover of the Maggies appears to be better designed in that it presents a more even keel impedance vs. freq.
Hope that this helps some. FWIW. IMHO. YMMV.
