Sensitivity, Watts and impedance: Speaker Matching

Which of these features is the worst enemy of low-wattage amplifiers (50W into 8ohm, 75 into 4ohm, like in my case). Should I steer away from 4ohm loads if my amp has between 50-75 watts? What if they are efficient (+90db) but low impedance?

I have in mind a pair of Audio Physic Tempo III but I don't know how good a match they may be with my Blue Circle CS.
I would stay away from speakers which have an impedence which droops much below 4 ohms. It sounds like your amp doesn't have a very high current capacity. If the impedence of the speaker remains above 4 ohms and its efficiency rating is at least 90db you would be alright except for very loud levels. Moderate levels should be no problem.
Hi, After doing a little research on the Audio Physic Tempo III I found these speakers will take 180 watts at 4 ohms.
From the reviews I read most people were using 150 to 200 watt at 8 ohm amps. It would seem that your amp may not have enough power.

In amps, when you reduce the resistance you make the amp work harder because at 8 ohms you get 50 but at 4 ohms you get 75 watts. The resistance is called nominal which means nominally at 4 ohms but it will vary a little bit depending on load so this speaker could go down to 3 ohms at times.

As Newbee states If the impedance of the speaker remains above 4 ohms... well these are 4 ohm speakers

Anyway I would just contact both Audio Physic and Blue Circle and ask them about matching these two products.
Audio Physic's web page was no help in this regard.
good Luck!!