speakers with a "smooth" impedance curve

If your speakers impedance doesn't swing too wildly and/or deeply below 4 ohms, then you may not need to worry unless you have a tube amp without 4 ohm taps.

But it all depends on how loudly you like to listen.

Now if you're talking about SET amp, that's a different story. SET tube amps have limited power & like smooth impedance curves above 8 ohms for the most part.

Read this article by Welbourne Labs (http://www.welbornelabs.com/recomendspeaks.htm). Be sure to scroll down & take particular notice of article "Some Insight into Proper Speaker Selection."

To Beavis

My current speakers have a nominal impedance rating at 6 ohms. I can't find an impedance curve on these speakers, but the manufacturer recommended the 8 ohm taps on my amp. I think the system sounds much better using the 4 ohm taps. The question is, if the impedance curve does drop below 4 ohms, does that strain the amp and risk damage to the amplifier?

>tower speakers in the less than $5000 price range

That was my price range 7 years ago. I bought Coincident Super Eclipse IIs in black. I have Welborne Lab 300B SETs. I love these speakers.

According to the manufacturer the load does not drop below 10 Ohms, and the sensitivity is around 93 dB/w/m. Your 40 watters would make the Super Eclipse sit up and beg.

Regards,

DeVore Fiidelity. I am running Gibbon Nines right now
w/ 75 watts. It is plenty of power.