The way amplifiers are sold is on power where power (in watts) = ( V x V ) / R, where V is the AC voltage (usually RMS rather than peak or peak-peak) and R is the resistance of the speaker.
Now, look at the specs of many amplifiers regarding power into load. For example, you might well see that an amplifier is capable of 60W into 8 ohms and 100W into 4 ohms.
Put these numbers into the above equation and things aren't adding up. An amplifier that is capable of 60W into 8 ohms should be capable of 120W into 4 ohms (as the output voltage of the amplifier remains constant, and is limited by the power rails of the amplifier). In other words, the VxV remains the same, R has halved so power should double.
Why doesn't it? Power is also equal to I x I x R, where I is the current ouput of the amplifier. Thus, if you have 100W into 4 ohms, I x I is 25, so the amplifier is putting out 5 Amps. 60W into 8 ohms gives I to be 3.5A. So, even without the power doubling, you can see that the current requirement of an amplifier into a 4 ohm load is significantly higher than into 8.
So what? I have 8 ohm speakers.... Do you indeed? The impedance trace of any speaker is not flat, due to resonant nodes (at which the driver may move freely) and anti-nodes in the cabinet. A port, for example, is a deliberately introduced resonant node. This causes the impedance trace to vary quite significantly at very low frequencies. Higher up the frequency range, the crossover network of the speaker will provide a varying impedance with frequency (that, afterall, is the job of the crossover network). A "nominal 8 ohm" speaker may, in fact, dip as low as 3 ohms somewhere in the bass region.
Coming back to the original question, Rotel amplifiers are unusual among budget amplifiers in having quite significant power supplies that will allow the amplifier to deliver significant current. Why is the RA-1062 only rated at 60W into 8 and 90W into 4 ohms then? Because of distortion. With a 4 ohm load most amplifiers will have slightly higher distortion than with an 8ohm (you can optimise for a 4 ohm load instead, but the majority of speakers that the 1062 is likely to be used with are 8 ohm). Rotel have provided both ratings at the same 0.03% distortion level.
Another issue is that of the output impedance of the amplifier. In the case of the rotel, that is 0.053 ohms (as the damping factor is quoted as 150 into 8 ohms). Generally speaking, the lower the output impedance, the better. Some of the big rotel power amps have output impedances as low as 0.008 ohms.
