When it comes to my body, this is what works...
F isolation crap altogether (preacher, concentration, etc... isolation DEF limits overload IMO)... don't overthink such a huge grip variety... only one thing makes my arms grow: OVERLOAD.
Just try this... see if it works for you, does for me...
3-4 sets straight bar curl (a few inches wider than shoulder width)
2 sets dumbbell curls
1 set hammer curls...
On form... all this stuff about being super stiff, that really limits my growth... I say DO sway... just make sure it is natural and not a forced swing just to get momentum... but don't make yourself stiff and limit overload.
On dumbbell curls (non-hammer)... bring ur arms down so that the backs of ur hands hit the front of your upper legs and get a good stretch.
#1 CONCERN... use the weight that will bring you to absolute failure either between 3-5 rep or 4-6 rep range... high rep ranges don't cut it... lift heavy. Don't limit your range of motion. Start the movement with arms fully extended and bent forward about 5-10 degrees. Again, allow swing, but keep it natural, not just momentum to lift extra weight.
Lemme know how it goes... some may disagree with me on this... but this DEFINATELY works for me, and nothing else does for me. I've gone up 20-25lbs on my barbell curls in just 2-3 months.
EDIT: One last thing... excessive warmup can limit overload also if you create any fatigue. Only warmup before the first exercise for the bicep... so in this workout plan for example, if you barbell curl 120... then only warmup before the barbell curls, and just do about 2-4 reps total at around 90 or 100lbs... then go straight to the weight you are aiming for for the next few sets.