Re: speaker rms...
That's going to depend on the mechanical and thermal design limitations of the sub. Some subs are underrated and can take massive amounts of power with no ill effects. Others are rated at about what the manufacturer says, and still others are overrated and can't take the power the manufacturer states.
As a general guideline, too much power is better than not enough, as you can always keep the amp gain set at a lower level to deliver less power than what the amp is capable of delivering to the sub.
And by subsonic frequency, do you mean the subsonic filter on the amp itself? All that does is prevent very low frequency signals from being passed on to the sub. For example, if you have a sub in a ported enclosure tuned to 33 Hz, and the amp has a SSF with a 24dB/Octave slope and it can be set variably between 10-50 Hz, then you'll want to set it about 20-25% lower than the tuning frequency.
2010 Kia Forte SX
2009 Kia Borrego EX 4x4
Setting Amp Gains:
Bought from: jacobcopeland87, 95bat, alxmlr789, na_rsx