Powers up but no signal

yes it very much has everything to with it. your sub out is at a pre-amp lvl, so you can route that signal to another amp, then to your passive sub, or get a powered sub

check your subs by connecting them straight from the stereo. If they work then there is an issue with the signal comming from the stereo through the rca cables to your amp, check those. Try another amp if you can with those rca's or test the amp with a friends car, either way try to rule out everything until it points to the problem.

Have you checked to see if the sub amp is turning on. Check to make sure you have at least 12 volts on your power as well as your remote turn on (Blue/White on radio harness).
If everything is hooked up correctly, it is possible that the preout is turned off. Some headunits have selectable preouts. If so you should be able to turn the sub on and off in the menu.
Hope this helps
VOTE

yes. A good idea. Kill power to the sub, then unplug and remove everything from the sub. go ahead and plug it in again and check for buzzzz.
That is test one. if you have buzz still, its the sub assembly. bad luck, but not impossible. If the buzz is gone its one of the signals to the sub. They can produce a buzzz without any signal coming from your stereo equipment. Replace the feed line, enjoy! BTW...Kef sure does sound clean. good choice.

check your fuse on the amp hopefully you didnt burn it did you brige 2 subs into 1 channel on your amp

you could have a bad ground or the power wire could have a short i would start there

Any idea how this is all wired up?
I think what they have may used is an 'RF modulator kit' to get the signal from your stereo to the amp. These are commonly used when factory stereo's don't have any outputs on the back of them for adding external amps. Even so, when you unplug the RCA cables there should be no signal coming to the amp & no sound coming out of the speakers/subs that are subsequently connected to the amp either.

is this a new problem or has it always done it?? it mos tlikely needs repairs, but try this first. Hook up ther + of one sub to the speaker + on the amp. Hook the - of that samne sub to the + of the other sub. Then connect the - of the other sub to the speaker - on the amp. This will run the speakers in series and will put alot less strain on your amplifgier. It will not "hit" as hard but it sbetter then having it shut of fall the time. I would also make damn sure that:
Your ground wire is large enough and properly grounded. It should be BOLTED to a solid poiece of metal on your car. Use a screwdriver, whatever so scrape off any rust as the connection should be a clean, BARE METAL connection. Next make sure your power wire and fuse are ok. If you are using round glkass fuses, replace it even if it looks good. I have seen those fuse smelt the solder in the ends. They can look great but really be bad.

SO go through all the basics and if it still gives you problems then it needs repaired. Please dont rate me un-helpful.

I am not familiar with your amp, but any amplifier performs best when the output stage effective resistance exactly matches the load resistance. So, read your 15" subs and find out number of ohms, then set the amp to that number. according to your amp's specs: if you have 2 ohm speakers, you can draw 350 watts from your amp. so purchase 2 ohm speakers

Make sure the amplifier setting "disable" is off then turn off remote power and turn back on.