Although I am of course not familiar with the internal design of the JL sub, I absolutely can't envision a way in which driving both inputs at once could cause damage to it, assuming that both inputs are not simultaneously overdriven with absurdly high volume signals. My guess is that the reason the manual refers to not using both inputs simultaneously relates to sonics.
First, with both inputs simultaneously connected to different components there conceivably could be an increased chance of ground loop issues arising.
Second, my guess would be that the unbalanced signal input is simply connected directly to one polarity of the balanced input. If so, that would mean that whichever component is providing the input signal that is being used would have to drive a load impedance approximately equal to the output impedance of the component that is connected to the other input. That will be a much lower impedance than line level output stages are normally called upon to drive, and, depending on the design of the particular components, compromised sonics might result. And I suppose that a poorly designed output stage in one of those components might even suffer degraded long-term reliability or damage as a result. Finally, it would mean that the balanced input is no longer truly balanced, because the two signal polarities would be loaded in a very unequal manner.
As far as line level switchboxes are concerned, the DB Systems DBP-2JAU/5 from DB Systems may be of interest. I've had occasion to use one in the past. It is nicely made and does what it is supposed to. It costs $110 for the version with gold-plated jacks. My one concern about that approach would, again, be the possibility of ground loop issues arising.
Regards,
-- Al
First, with both inputs simultaneously connected to different components there conceivably could be an increased chance of ground loop issues arising.
Second, my guess would be that the unbalanced signal input is simply connected directly to one polarity of the balanced input. If so, that would mean that whichever component is providing the input signal that is being used would have to drive a load impedance approximately equal to the output impedance of the component that is connected to the other input. That will be a much lower impedance than line level output stages are normally called upon to drive, and, depending on the design of the particular components, compromised sonics might result. And I suppose that a poorly designed output stage in one of those components might even suffer degraded long-term reliability or damage as a result. Finally, it would mean that the balanced input is no longer truly balanced, because the two signal polarities would be loaded in a very unequal manner.
As far as line level switchboxes are concerned, the DB Systems DBP-2JAU/5 from DB Systems may be of interest. I've had occasion to use one in the past. It is nicely made and does what it is supposed to. It costs $110 for the version with gold-plated jacks. My one concern about that approach would, again, be the possibility of ground loop issues arising.
Regards,
-- Al