I don't know for certain about this particular model, but Hartke does use "maximum watts" in model naming, website spec and ad/promotional text, which is always double what they give as RMS (when they give it), so this may all be moot.
I haven't used the 400RB, but I've used the 1001RB a few times and was very impressed with the sound and feel. From all I understand, "familial sound wise" the line has the same well known and highly regarded sound. I've used amps in various situations from 60 watts to 1,000 watts. This is a piece of pro gear, and running 2 x 12s at 4 ohms, personally, I have difficulty envision real world scenarios for myself where I could see feeling the need for just more power from the amp without other changes (like more speakers).
As often as people discuss the relationship between loudness and power here, it seems far less often that this understanding gets applied to discussions of this type of decision. That relationship is logarithmic where doubling of amp power creates a 3dB increase in loudness, and a 10dB increase is required to create a doubling of loudness.
If you are hypothetically using about half of the total capacity for loudness of an amp with a maximum output of 600 watts, the power output is around 60 watts. Now, just as amps don't have wattage gauges or dB meters, they don't have percentage of max loudness gauges either, but just in terms of some of the fairly easy to understand math, I think being number focused with wattage makes less sense. Personally, I've never run any amp I've owned at flat out full power, and it's never been the norm to set up and simply turn my amp up until it feels like I can't anymore. I realize that people really do face situations that are beyond an amp's capabilities, but my limits on output have always been what's appropriate for the situation, not turning the amp up until it has no more to give.
I can't tell you by ear precisely when a -3dB change in sound has occurred, but I think for this purpose a rough estimate works. If you assumed just for the sake of argument that the loudest you actually can/have ever turned the 600 watt amp up to is full power, then imagine turning that 600 watt amp up as loud as it can go and then turning it down by a noticeable amount and then another noticeable amount from there...it's very rough, but if the starting point really was 600 watts (which it probably wasn't really), then you'd likely be in the range of 100-200 watts. Getting ~150 watts from a 280 watt amp can be done just as readily as getting ~150 watts from a 600 watt amp.
IMO, given these two amps and the same 212 speaker setup (and admittedly having no knowledge of how your specific situations differ from mine), if the need for volume was beyond the 400RB it'd be likely that more speakers would be a greater benefit than the 600 watt amp.
