And we can make the discussion even more confusing (possibly to the point of being meaningless),... by bringing into the equation a speaker's power ratings and impedance curves.
First power ratings.
Let's say a speaker has a rating of 150 watts. That means it can handle 150 continuously without over-heating the voice-coils (usually referred to as "Thermal Power Handling"). But that doesn't mean a 175 watt amp is too big. That same speaker will be able to handle peaks for a moment of perhaps 250 watts. Basically,... short term it can handle power up to the point where the cones bottom out.
So you want an amp that can drive the speaker to proper volumes,... and have enough in reserve to send out those peaks when needed. When I build car stereos,.. I usually try to size an amp at around 1.5 times the speaker's rated power handling. Better to have a woofer cone go "clunk" once in a great while than clip the amp and smoke the voice-coil. I'd say 90+ percent of woofers are blown because of too little power,... not too much.
Next is impedance curve.
If you look at an impedance graph of a driver or complete speaker,... you'll notice that it is only really a 4 or 8-ohm speaker at one frequency. Here's a graph as an example.
As you can see,... this 3-4 ohm speaker is really only 3-ohm at 15,000 hz. At other frequencies it is as high as 33 ohm.
This means that when your amp is playing music,.... it never really sees 8-ohm loads except during the moments that is is playing that particular frequency that is 8-ohm. The rest of the time it's just loafing.
Here is a graph of a 10" Eminence 6-ohm woofer.
The fine line is the impedance curve,... and the ohms are listed on the right.
As you can see,... the impedance varies from 6-ohms at 220 hz to nearly 100 ohms at 52 hz. If it were crossed over at 150 hz,.. the impedance of this 6-ohm woofer would range from 8-ohm to nearly 100-ohm.
In the end,.... it's a bit of a mess. You won't really know if you're short on power unless you hear the speakers break up at a volume that is too quiet for your needs,... and you won't know if you like an amp's sound until you try it in your own system. That's why Emo's 30 day risk free trials are such a fantastic tool for the customer.