I don't think it's a dumb question at all. It's a question that can have a lot of different answers, though.

For the Model 755 or any other amp with only unbalanced inputs, there isn't really a question at all - unbalanced is the only option. If you own an amp with balanced and unbalanced inputs, I would suggest trying both and deciding which you prefer. If you are planning to buy an amp at the same time you buy the 990, then you have the option of buying an amp with unbalanced connections only (which will typically be less expensive) or buying one with both balanced and unbalanced connections. At that point, all I would suggest is that you try to identify a few amps that you might be interested in and demo them to see if they fit your needs. For what it's worth, I expect that most 990 owners will use unbalanced. I know I will (my amps only have unbalanced inputs), and I will do so with no qualms about it.