Post from lilmike on the subject on a different thread:
Power handling is related to the driver and cabinet combination. There are two limits, thermal and excursion. Thermal is simple - how much power does it take for the glue and varnish on the coil to melt. Been there, done that, don't recommend it. Excursion is more complicated, as it is sort of related to power, because more power = more excursion. It is also related to driver design - some drivers can handle it, some can not. It is also related to how you load the driver - what sort of cabinet is it in.
Excursion limits are typically discussed with two numbers, given in millimeters.
xmax = linear excursion, which is established by the driver's motor design or measured performance. Things tend to start sounding bad at this point, but you're not damaging anything by crossing this limit.
MFW = 13.02 mm per Eminence specs
RSS390HF = 14 mm per Parts Express
xmech = the point where moving parts hit stationary parts. Things go badly here - coils go clank, suspension makes noise, tinsel leads slap or break. This is a bad place to be, the driver may be irreparably damaged by exceeding this limit.
MFW = 20 mm per Mark Seaton, but this is suspension noise, not collision
RSS390HF = ?, but a web search suggests that 20 mm is appropriate
We can reach the power limit first, or we can reach the excursion limit first. That depends on how we load the driver. Simulations are invaluable here, but real world testing is what matters most, because the simulations I generally use are "nonresonant" and "lossless", whereas the real world is resonant and lossy. Simulations suggest that at 300 watts, I should be exceeding the point where the MFW-15 makes significant suspension noise. When I tested the MFW in the F-20, I did not reach the mechanical limit of the driver, and I exceeded the thermal limit by 33%. I certainly exceeded xmax, but I did not hear any complaints during high power sine wave testing. Honestly - I did not exactly wait around for it to happen, but we were looking for it and did not notice. High power sine waves are something that will kill drivers dead if done cavalierly. Hence - I concluded that when used in this cabinet, a 300 watt amp is a great match for the MFW 15, when used with a highpass at 20 Hz.
The F-20 was about getting the most out of the least. I targeted a 115 dB SPL at 20 Hz, measured at 1M in a groundplane setting, then worked backwards from there. I used the MFW because they were available, and provide a LOT of displacement for the dollar. I got a driver, measured the Thiele-Small parameters to make sure they were consistent with what was provided by Eminence, then modeled a bass horn to meet my goals. I folded the design into the simplest horn I could while minimizing the compromises wherever possible. I sacrificed size, and I limited bandwidth. Something had to give, I wanted low and loud, so I accepted large and I gave up the subsonics because huge was out of the question.
The fact that other drivers work at all in this cabinet is simply a matter of coincidence. The fact that a number do is a very happy coincidence. I'd power the RSS390HF with 300 watts personally. I'd also recommend a highpass at 20 hz, because in my opinion, applying watts of amplification outside of the operating bandwidth of a bandwidth-limited cabinet is a bad idea if you're after accurate, low-distortion reproduction.
SPL? Can't tell you what your room will give you, but four, in a cluster, outdoors in a groundplane setting should deliver in excess of 125 dB at a meter when fed with 300 watts each. This may not be enough for a huge room, but more power won't help things.
Funny thing about adding more power - it is a logarithmic progression. 1 watt to 300 watts nets you 24.75 dB of SPL increase. According to the math, increasing from 300 to 500 watts gets you 2.2 more dB, but exposes you to burnt or clanked coils, and does not account for losses due to voicecoil heating. A lot more risk for very little benefit in my opinion.
I think 300 watts will be safe based on the simulations, but I have not actually tested the RSS390HF in this cabinet, nor have I measured the Thiele-Small parameters of the RSS390HF. Comparing modeled results at 300 watts suggests that at the same drive voltage, the RSS390HF (gray) has a lower excursion than the MFW (black).