you are not being stupid, its just you are making the same mistakes as most people
That amp ( although the link doesn't work) I promise you will be 2 ohm stable in stereo mode so don't worry about running the JBL's if you want. In fact I just found a link and I had that exact amp so I
know for a fact it is 2 ohm stereo stable.
if you ever fancy selling that amp then I would ahppily buy it off you, I am dying to get my hands on another one.
if you use the tweeter then
this will mess up the sound stage, it will sound like the sound is coming from everywhere which is not how it should be.
running without the tweeter will lessen this negative effect but not eliminate it.