Welcome to the forum. I could find no mention of the frequency response of the Pioneer. Very odd. There could be a number of issues here. First, your signal you tapped into for your RCAs could be filtered and not passing the lower frequencies. Your RCA adapter could be suspect. Cheaper ones usually don't work too well and given the amplifier on the Pioneer has speaker level inputs, you'd be best off using those. It's just one less thing to go wrong. You could also have the crossover on the Pioneer set incorrectly, a cancellation issue, or it just simply may not dig that low. I'd start by checking the filter on the amp and adjusting it all the way to the right. See what happens. If that does nothing, I'd move the tube around to rule out cancellation issues, if that doesn't work, remove the adapter. You see where I'm going with this?