From a purely theoretical perspective it is likely very true that a capacitor would degrade the Q of a perfect coil. In many practical applications the Q of the matching coil might not be the determining factor of the sucess of the total assembly.
This is because the bandwidth of such a ridiculously short antenna (3 meters for the AM BCB) is so impossibly narrow that the possibility of achieving a match with the coil alone is nearly impossible for the average experimenter.
While adding an air cap will likely degrade Q, the ability to fine tune the total assembly for a better match sometimes results in better range in spite of the reduced Q.
This is why the air cap is an often-recommended tuning aid - it's not perfect, in fact it might even be a detriment to one side of the equation, but it can sometimes help the situation in total - a beneficial compromise, so to speak.
One of my favorite quotes - "Don't let perfect be the enemy of good"
Thanks for all the help.
I think I'm going to try tuning at the lower point, then raise it on the mast. I tried it with the new coil (more about that in another thread) at the lower point. With no tuning at all, it got out 1500 feet in some areas, on 1630kHz, which is where I designed the system to work. Not too bad. I'm going ahead with tuning the antenna. then raise it on the mast and see what I get.