Balanced But Not Fully Balanced

I own a preamp which has both balanced and single ended jacks. I assumed that since I was using the balanced jacks I was getting the benefit of a balanced circuit. I have just now realized that just having balanced plug-ins doesn't mean your preamp (or any other component) is "balanced." Just wondering what sonic compromises are being made with equipment which has balanced inputs and outputs but changes the signal to single ended as it passes through it. If you are using the balanced outputs, what good is that if the signal going into this jack is single ended? I don't understand what good is it to offer equipment with balanced capability only to revert to single ended signals. Is this just a gimmick to sell equipment or is there some advantage to not making the circuitry "fully balanced?"
Regardless of whether or not the component has a balanced internal signal path (i.e., is "fully balanced"), a well designed balanced interface will reduce susceptibility to ground loop effects; will to some degree reject noise/RFI/EMI that may be picked up in the cable; and may also reduce sensitivity to cable effects and cable differences.

Having a fully balanced internal signal path provides additional benefits that are essentially unrelated, including reduction of certain forms of distortion.

However, the potential advantages of using a balanced interface between components having unbalanced internal signal paths will in some cases be outweighed by adverse sonic effects of the circuitry required to convert the unbalanced internal signals to the balanced signals that are provided to output connectors, and to convert the balanced signals at input connectors to unbalanced form. Also, I suppose that at least a few highly compromised designs may exist that don't bother to include any conversion circuitry, and instead provide unbalanced signals to XLR connectors, which would negate the potential advantages of a balanced interface.

So if the components provide both choices, and are not fully balanced, the only way to determine which interface will perform best in your particular setup is probably to try both of them.

-- Al
If the gear uses transformers for input or output, then it can operate fine in the balanced domain with no tradeoffs. But preamps with input or output transformers are rather unusual.

Balanced operation was originally conceived to eliminate problems with interconnect cables and it does that very well, in fact so well that it does not matter if the equipment is balanced or not if it can drive the cable correctly- the difference is that profound. Its how the record industry was able to hang mics in a concert hall 50 years ago and at the other end of 200 feet it still sounds like music :)

But a lot of companies just install the XLR cable to be trendy. If that is what you have it will probably work no better than the RCA connection.