I agree..SPECS will tell all.
the distortion can be anything from not enough power on input side that the amp cant efficiently boost it. The Signal to noise ratio (SNR) and total harmonic distortion (THD) or (Db) as a refrenced also.. the higher this number the more music it reproduces at a lower per volt input. These can be the difference in high quality vs low quality amps. The S.N.R. is where you want to focus for input side as this is where the amp tells you how well it reproduces and amplifies the original "music" without degration. the THD is how much distortion is heard on higher volume for instance a THD 1% would in perfect world be 1% is pure distortion @ 100 percent not including enhanced effects, so this you want as low as you can go. It's always my first look when buying amps..
Also it can also be wrong wiring type for the amp.. it sounds like your running it 2 ohm load but this can be wrong assumption.. if your amp isnt rated for 2ohm load then it will kick off upon higher volume as this is when the amp's transistors are being strangled, smacked and beaten.. if you had it wired in series your amp wouldnt cut. try unhooking one and see if it still cuts? I bet it don't.. this will tell you if you need to change wiring from parallel to series or buy an amp that supports 2-ohm loads. Another possible problem but unlikly with only 240 watts can be your amp has a protection for low battery voltage when the bass kicks the the amp voltage drops and temporarily goes into protect mode this can be also the case for wrong wiring for the subs as earlier mentioned... if the amp isnt rated for it! Im not sure the output of Xenark screen output but I can asure these Jl boosters will make your input much more cleaner and stronger. I ordered a pair of these yesterday. But rest assure JL leads in automotive audio in SEMA, my wallet proves this..lol
I am willing to make a friendly bet that your amp shuts down because its not rated for your Ohm load and the distortion is too low voltage level input all this points to a low quality amp