I would email NAD. I am guessing the 8 OHM minimum is more of a guideline than absolute rule. Solid State amps love a lower impedance in general...
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A 4 ohm resistor in series will soak up half the power sent to the speaker and release it as heat. You may not be able to get enough volume out of the system. Your resistor also needs to handle the power. I'd suggest a power rating of at least P*.707*.5, where P is the receiver rated max power.
If you hook up the speakers directly the receiver may run hot, its protection circuitry may trip, or at worst case you could damage the output stage.
A resistor is not a good solution. Besides wasting a lot of power, it will cause the frequency response of the speakers to deviate from flatness in proportion to the variations in the impedance vs. frequency curve of the speaker. It will also severely reduce damping factor, degrading bass performance.
I notice that Audio Advisor sells a package consisting of the L54 together with PSB speakers which are specified as 6 ohms nominal, 4 ohms minimum. Also, it would be unusual for an amp or receiver to be designed to truly be limited to 8 ohm speakers. I suspect that calling NAD and also Googling to see what other speakers people have used with the L54 may end up supporting the conclusion that the manual is incorrect.
My brother was "sold" a reciever with such a warning as part of a package with speakers that were a nominal 4 Ohm load. Needless to say, at anything near loud volume, the the reciever would cut out. These "salesmen" should be ashamed of themselves. Rather than selling the speakers, I would suggest returning the reciever.
I don't know if this is useful in your situation but I used to have a pair of older Monitor Audio 4-ohm speakers that worked quite well with a Parasound amp. When I switched speakers I sold the Monitor Audio's to a friend who planned to use them in his new house.
When he hooked them up to his inexpensive multichannel receiver it almost immediately shut down every time he turned it on. Fortunately, the receiver had an overload protection circuit. I went over to have a look and noticed he was using 18 gauge lamp cord for speaker cable. When we replaced it with the same 12 gauge cable I had been using it worked fine and he's still using the combination several years later. The speakers were just pulling too much juice through too small a wire and the resistance was causing major overheating.
I well remember a "Professional" sound man using at least 75' of 20 gauge wire.
Of course, Stan and El are BOTH right. The added resistance of speaker cables which are too small a gauge and/or very long would make life easier for the amplifier, by virtue of increasing the load resistance it sees, but would waste power and degrade sonics, particularly by reducing damping factor.
>Stanwal...AWG 20 wire is 0.0119 ohms per foot.
150 ft of such wire (75ft round trip) would be 1.78 ohms.
Not really a problem, especially if the speaker is 8 ohms.
Sure, but that "8 Ohm" speaker can drop to 6 Ohms at some frequencies and increase to 32 at others.
Attenuation will vary from -.47dB to -2.25dB at others, and a 1.8dB difference in total power response is going to completely change the speaker's voicing.