Fosgate p325.1 amp comes on and appears to be

turn down the gain until you can go full volume without distortion.
crossover frequency is always up to the user.

low pass of course set the crossover around 80Hz. Then turn the bass down to around 35% on headunit and the volume all the way up on your head unit just below distortion. slowly turn the amp gain up until you start to hear distortion, then turn it back down until it's completely gone. Then, lower the gain just a little more to make sure you keep out the distortion on all songs. From here you should be good. If the gain is still to high. The adjust to your taste. Just don't move the bass up on the headunit or the gain past your distortion point.

I’m happy to help further over the phone at https://www.6ya.com/expert/chris_7f7f91eb941fb23f

Bass boost simply adds DB or gain to the bass signal. Some like it, some dont.
The frequency knob is the Low Pass Filter (LPF) that cuts off the amp output above
the set frequency. A bass system usually cuts off below 400 HZ and more often 200HZ.
A good, rattle your car bass note is in the 25-80HZ range or lower but most amps and audio
input devices cant attain a good low note with clarity.
"Low-pass filter allows only frequencies below the crossover point to be amplified. A high-pass filter allows only frequencies above the crossover point to be amplified - useful for keeping low bass away from small speakers, so they can play more efficiently."
Checkout Crutchfields Glossary of Car Amps.
http://www.crutchfield.com/S-LPyoWWgD5qN/learn/learningcenter/car/amplifiers_glossary.html

Did you use the installation manual?

You can buy a decent hi to low crossover box from any stereo shop, peripheral makes em, so does rockford fosgate, even scoshe at walmart. The new rockford fosgate 3 sixty is pretty neat but I don't know the price.

check the internal fuses inside the amp its self these burn up real easy plus if it is them it will save you some money

It all depends on the type of music you listen to. I fyou listen to music that consistently produce low frequency sound then you would want to enhance those frequencies with an EQ and reject unwanted tones with your crossover network. It is all personal preference though. If you think it already sounds good then why mess with it.

If your amp has rca jack inputs only you will need in-line filters to reduce the stock stereo speaker outputs down to less than 3 watts. If your amp has high power inputs you can use those. If your amp does not have a built-in crossover then you will need to install a low-pass crossover or filter before the amp so the only the bass comes thru or the sub woofer will have major distortion. Another problem is the amplifier needs a turn-on signal for the blue wire and most factory radios do not have one. That means you will have to tie that wire into the switched 12v wire from the vehicle so the amp comes on when the key is switched on. It can be done be done but I don't think you will be very impressed with the sub due to most factory stereos have a limited bass range. Most aftermarket head units have built-in crossovers and sub outputs. They also produce much better sound than the factory unit. So in other words yes you can.

1800W peak output at 2 ohms
975W peak output at 4 ohms
600W RMS (continuous) at 2 ohms
325W RMS (continuous) at 4 ohms
Variable LP crossover adjustable 35-250Hz
4 AWG power/ground inputs and RCA pass through
Low-pass filter
Bass boost 0-18dB adjustable remote punch bass EQ
Signal-to-noise ratio: >80dB
Line-level input


It is going into protect mode for a reason.
Disconnect the subs and turn everything on again. If the light comes on, the problem is internal and will need servicing by a technician.

you should be able to remove the passive in-line crossover, but since it is blocking everything below 3500 hz, there is no need. leaving the passive crossover in place will also help protect the tweeters if something fails in your setup as any amped frequency below 3500 can seriously damage them.