Some vinyl triggering sub to crackle

The sub should never crackle. I can not tell from your description exactly what is happening. It could be a mechanical problem with the driver. I has a loose dust cap on one sub. It "burped" with certain notes. A little glue fixed it. It could also be the subs amp being overloaded or the driver over driven. A subsonic filter rolling off sharply from 18 Hz is always a good idea with subs but the best way to do it is digitally and most do not have that capability. Analog filters do not roll off fast enough and frequently cause sonic issues with the bass. Turntable set up is very important. You want to make sure your resonance frequency is at 10 Hz or so. Even set up correctly you will see some woofer movement. Just goes with the territory. If you switch over to a digital preamp like the Anthem STR, DEQX or Trinnov Amethyst you will gain total control of the situation. But, you need to rule out a defective woofer first. You should have two woofers anyway so why not buy another Rel. If it does not crackle then you know you have a defective sub which needs to be fixed. 

It may be gone by then. You really need to get a active crossover and stop connecting the subs to the amp. I know that is what REL recommends but they are doing that to lower costs to make their subs more appealing. I just had an experience with a fellow who was using a Parasound Hint Integrated which has a built in subwoofer crossover. H was use in the amps output to drive the subwoofers and was unhappy with the sound. I got him to move the subwoofers to between the speakers and to hook their line inputs to the hints crossover adjusting the crossover appropriately. He says he was blown away by the difference. This was all done by email so I could not listen to it personally.
Trying to get away without a proper subwoofer crossover partially defeats the advantage of getting some warfarin in the first place. But, the subwoofer manufactures do not want to subject the purchaser to the expense of having to buy another 2 sets of long interconnections. They could defeat this problem by providing outboard crossovers for their subwoofers.  To my knowledge only JL Audio does this.  The fallback position is to install the appropriately sized capacitor on the input of the amplifier to roll off its base. This is only first order right better than nothing.