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.