I think i know what you need this amp for
When talking spl, when used in a human hearing context, spl is weighted with an A weighting, where low frequencies are heavily
attenuated (20dB down at 100Hz, 40dB down at 20Hz). This is due to the fact our hearing is incredibly insensitive at low frequencies compared to midrange. http://en.wikipedia.org/wiki/A-weighting
However, if you're trying to produce low frequencies from an amp/speaker, well that's all "unweighted" spl you're trying to produce, so trying to get 100dB of perceivable bass (a weighted) needs 120dB or more of raw spl. I might be wording this in a funny way, but i hope you pick up on what i'm trying to say. It's also why you can find a 10 or 20W guitar rig can keep up with a 400W bass rig.
Generally, it's better to over-rate amps compared to speakers than use under-rated ones, as clipped
peaks from an under-rated amp do more damage to speakers than a peak above their rated power. But if you're not driving the amps or speakers at, or near max, then no need to worry here.
So in other words for your purposes, don't stinge on the amp.
But given every time you double/halve output power is only +/- 3dB in headroom due to the logarithmic nature of our hearing, there's not really that much difference between a 300W and 600W amp, so you probably could get away with a 300W amp, but i wouldn't go lower.