AFAIK, the only potential issue with splitters is that the receiver's output sees a lower input impedance because the multiple amp inputs are connected in parallel. But again AFAIK, the primary potential issue is high frequency roll off, and thesea are subs we're talking about, so if the output were "massively" down by 3 dB at 10 KHZ, it's so far out of the relevant passband as not to matter.
Lots of folks here do it. The pre is only swinging a volt or 2, and even if you've pulled the input impedance all the way down to 250 ohms, that's only .008 amps of current, or a total of 0.016 watts . . .
First off I would highly discourage using the "subwoofer" in your center speaker. It is not a true subwoofer but just a woofer that is used to extend the center frequency down to about 40Hz (and that's on a good day). With your current receivers universal crossover for all channels I would suggest hooking all speakers up using speaker wire only hook the sub using the sub preout using RCA jacks and set the pioneers crossover to 40Hz. Give that a try. If you notice distortion during bass heavy movies raise the crossover to 60Hz. DefTechs built in "subs" are not true subwoofers and are best described as "mid bass" subs meaning they work best at 40-80Hz anything lower and the bass becomes boomy and distorted. I know I have BP7001's and when set to Large the bass sounds horrible. Set to 40Hz and the bass is very tight loud and well controlled. I let my SVS handle everything below 40Hz.