I have a question as well and it seems like it might be the same issue. I bought a sony head unit that is rated at 20 rms and I replaced the two rear speakers with a pair pioneer TS-A6976R 550 watt speakers. Obviously they are overpowered but I got them brand new for 35 bucks so I took the deal. I noticed that they sound like they are heavily lacking in bass, but only when I have both connected. If I disconnect one, the other speaker seems to gain a tremendously large amount of bass that is gone when both are connected. Idk if it's the head unit or if I'm just underpowering the speakers. I'm assuming that maybe there isn't enough power going to both so if only one is connected, all the power goes to that one, but I'm not sure what the issue really is.
It sounds like you got a good deal on those Pioneer speakers! The issue you're experiencing with the lack of bass when both speakers are connected could be related to the power distribution from your head unit. Since the Sony head unit is rated at 20 RMS, it might not be providing enough power to drive both speakers adequately, leading to a perceived lack of bass.
When you disconnect one speaker and notice a significant increase in bass from the remaining connected speaker, it suggests that the power is indeed being distributed differently in that scenario. It's possible that with both speakers connected, the power is divided between them, causing a reduction in bass output.
You might want to check if there are any settings on your head unit that allow you to adjust the power distribution or bass levels. Additionally, considering the power difference between the head unit and the speakers, an amplifier could help provide more power to drive both speakers effectively, potentially improving the bass performance.