You're asking about flat speakers, but I think your premise is flawed right from the get go. This is because chances are better than not that your tapco speakers which you believe be flat are in fact anything but flat. A spectrum analyzer and a very good reference mic would tell you for sure but most of us don't have access to those kinds of things.
Very few speakers are truly flat and even if they are, as has been mentioned, the room you put them in can change that completely. What you're really looking for is speakers that are 'uncolored' i.e. speakers that don't try to artificially alter the source material in some kind of pleasing way. Once you find those, you have to put them in the room you're going to use them in and then you have to 'learn' them. Which means you have to start building mixes on them and then take those mixes and play them on as many other systems as possible to see how accurately they translate on the whole.
Once you do that, you learn what kind of compensation, if any, you need to make when you mixing i.e. less punch than you ultimately want in the low end or more sizzle than you really want in the highs etc.
Obviously the ideal is to not have to do any compensation at all. But those systems/rooms are rare. If you can get it dialed in to where you're making only very minor compensations, call it good and get to work IMO.