Hello EdD,
The short answer to your question is that in terms of power demands on your receiver's five internal amplifiers, the bass frequencies demand by far the greatest proportion of watts.
Think about it: the woofers in your M60s, VP150, and QS-8's have the largest cone areas and must move the greatest volume of air to generate audible bass output, hence they have the biggest magnets and voice coils (the speakers' motors).
This the best reason of all to use a subwoofer. Then you have a power amp (typically of at least 100 watts output) entirely separate from your receiver and dedicated to the sole purpose of moving a big woofer and generating all those power-devouring bass frequencies.
This leaves the woofers in the M60's, VP150, and QS8s to handle all the upper bass above 100 Hz, and your receivers' more modest internal amps will have ample power to do that without excessive current flow and possible overheating--even with the lower impedances of the VP150 and QS8s. The latter might only occur at very high volume levels using your receiver without a subwoofer.
By the way, most power ratings of many A/V receivers are somewhat misleading. The power is usually measured and rated with two or three channels driven, from 20 Hz to 20 kHz, into 8 ohms at such and such a level of distortion. It's rarely measured with all five channels driven across that frequency range because that would impose an extremely heavy--and unrealistic in terms of real home theater use--load on the receiver's power supply. So while your receiver may deliver 70 watts per channel with one or two channels driven, you can assume it's not able to produce that output power to all five channels simultaneously without a major sag in the power supply or thermal shut-down (part of the protection circuitry).
But keep in mind that playing music or 5.1-channel home theater material, your receiver would virtually never have to deliver equal power into all five channels simultaneously, because music, dialog, and effects are quite different in their power demands.
If, on the other hand, your receiver's power is actually quoted as 70 watts per channel into 8 ohms simultaneously with all five channels driven, then count your lucky stars (most aren't).
Lower impedances make greater demands on an amplifier's heat sinking and power supply because more current flows through the output stage--there's less resistance at 4 ohms than at 8 ohms--so the power-output transistors run hotter and at higher levels of distortion. However, as I stated above, this would come into play only if your receiver were used at very high volume levels without a subwoofer.
Regards,
