Paul, that 60w rms is the max. You don't need full power to get sound. Your 2nd question, of course you will notice the difference in power, but it'll sound just as good with 40w. Just about any amp will sound better than the HU at high volume. Don't get fooled by HU's power numbers. It may say 50w x 4, but it reality, it's 15w - 18w. They also distort badly at high volume.
yup power ratings are "maximum thermal tolerance" ratings, which is how they should really be labeled IMHO.. but anyway, you usually don't need full RMS power to speakers. 50 watts per channel from a good amplifier would be plenty.
also, difference between 40 and 60 watts. not a big difference.
remember, twice the audible volume is +6 to +10dB twice the power is +3dB you're talking less than double the power increase, so maybe +1 to +2dB which is considered "barely audible" in change of volume.
Try getting some speakers with higher sensitivity, so it doesnt require so much power. Like if you can return those and pick up some infinity speakers or something not much more in price but theyll do better with less power. I think anyways, im still new and learning, but this is what ive read. When you have the funds to add an amp you won't need a high wattage amp either which will save ya there, or save ya from the mistake of getting a cheaper amp @ higher wattage then going for a higher quality amp @ lower wattage.