I have an Alpine m110 amp rated for 1100w at 2ohm . It has been running a rockforg fosgate 12 inch sub rated at 650 rms and 12oo peak
I blew my sub because the amp was very powerful for 650 sub and 1100 amp
I just bought a 4 ohm 12" sub that is 1200rms and 2400peak twice capacity as 1st sub. All im changing is swirching subs. Should I touch the gain at all and what about lowpass. Ive heard you can burn up the equipment. Is there a place to check where i need settings. Use hz frpm sub specs. Should i just try as is. Or do i adjust since new sub can handle 2x pwr

I honestly would try it as is. You also don't want to blow out and speakers of the amp if it cannot handle the "tweaks" you want to make to the system.

Test it out always on lower settings and gradually change it.

usually the sub has all the specs on it try starting there and if you don't like adjust higher...if you want to blow it run max peak for a week

welxome to D2jsp i would suggest you to read the
D2JSP New User Guide
And the Official Site Rules and Policies seeing as your new here gl hf