Alpine MRX-M110 Amp hooked up to 2 Alpine SWS 15' Subwoofers.
Had these in for about 2 months now. Gave it easy for the first month. 0 gain for the first 2 weeks. Turned up to maybe 1/8 gain for 3 weeks. Now its at 1/4 gain and has been like that ever since. But I have 2 other gauges that I really dont know what they do.

Subsonic: 5-30hz
LP Filter: 50-400hz

I want to turn my amp up to at least 1/2 gain but I dont know what these should be set as to match it.

Can anyone tell me what exactly these options do and what they should be set as.

The last gauge is Bass EQ but I keep that set at 0. Is this the right thing to do?

sub sonic is how low it will play (given slope)
i usually have that turned all the way down but best advise is tune it to where your box is tuned to (if you got sealed leave it all the way down)

lpf is how high they will play thats a bit tricky to do but id say keep it around 60-80hz (just a hair off 50)

your gain shouldnt be adjusted according to 1/4-3/4s

its to be tuned with your headunit RCA preout voltage

if you have a 4v preout you shouldnt be at half gain
if you have 2v preouts half is pushing clip (depending on deck volume/eq settings)

subsonic is generally set half an octave below a ported boxes frequency. 32=X for this example and X=your port frequency when you do this for your setup. 32hz/2=16hz, 16hz/2=8hz. then you remove the 8hz from the initial box tuning: 32hz-8hz=24hz. that 24hz is what you would tune the subsonic filter to in this example.

on the low pass filter(LP) its really up to you and your musical taste. for example 80hz=standard but if you listen to stuff like metal music you'll want it more around 100-120hz.

bass eq should just be left at 0 due to it adding a sort of guess factor to how high you can safely set it.

now the important thing you should get outside help on(unless you want to buy the tool and learn for yourself) is setting the gain correctly. you will either need to find a shop that offers oscilloscope tuning or buy your own o-scope and do it yourself. the o-scope lets you see clipping so you can set the gain knowing if it is clipping or not at whatever you set it to.

This is the deck.
pioneer deh-x8500bh. 4v.
The box is ported. What do you think my gauges should be set at?

its also going to depend on how loud your door speakers get, you want your 1100 rms to peak with your door speakers max volume so you know how loud you can go safely for the whole system, witha pioneer, if you listen to around 30 volume max, i would say set gain to 1/4 nominal ( nominal is center on alpine amps usually designated by nom) and subsonic set to 20-23 hz for those subs, the box and sub wont play it but it will be comfortable with the flex. low pass filter st at 80-100 depending on your music. 80 for rap, 100 for rock. may need to be tweeked according to your application but good starting point. the amp supplies enough power ithout the need for bass boost or bass eq, however, if you listen to rap, the bass eq feature my provide you with the extra hard sounding bass if your dec does not support subwoofer level, but i believe it does.