Adjusting the 15" speaker tweeter may not do that much as you have 2 X 10" speakers above it which will contribute more edge to the sound. You do realise the tweeter control is fairly subtle, as you speak as if you think it is for the 15" speaker itself? TBH if you cannot get a good sound out of that rig I'd be very surprised. Have you had someone else play it and you stand 30' away?
Also your Gain looks as if it's at 10.00 o'clock. Have you tried it higher and has that livened up the sound? You can always dial back on the Master volume to compensate.
Good luck, I'm sure others will be along to offer their input....