Shaker 500 Subs w/ Aftermarket Head Unit Odd Question

Hello! I have an odd situation that I am hoping you can help me better understand. I will try my best to describe what my car's system is doing as I don't know much about car audio.

I have a 2006 GT 'vert with with the Shaker 500 8" door subs. I am in the process of gradually stripping out the factory system and going aftermarket. The subs and amps are the only things left. Not too long ago, I had a Kenwood DPX501BT installed to replace the factory head unit that decided to permanently eat a disk. Everything worked great, except I couldn't use the Kenwood's crossover and sub controls. The mobile install place said I wouldn't be able to use those controls until I got rid of the factory amps. Recently, I replaced all the 6 x 8 door speakers with Pioneer TS-A6885R's, front and back. My b/f installed the 6 x8's as they were a simple swap from the factory ones. Everything sounded great. However, I noticed something weird. I'm not sure if it was doing it before, or I'm just noticing it now because of the better door speakers. I noticed that the right subwoofer is being funny. With the balance at center, both subs fire. With the balance all the way to the left, both subs still fire. However, with the balance all the way to the right, no subs fire. Is that normal? Is this something I should bring up to the mobile install place? It's not like I would ever keep the balance off-center, but I just thought it was weird.

Also, when I get the factory subs/amps replaced, I'm thinking about going with the direct replacement/upgrade Kicker Shelby 8" subs. I've heard good things about them and they don't require any sort of modification to the doors. I know the factory amps won't be able to drive them which is fine. I will have the place install a new amp at the same time.

Thank you for all your help!