My biggest a loads are with prostate massage. I've never cum from just that but it's a new goal. ;-) I'd also like to separate orgasm from ejaculation this year.
I've been working my prostate occasionally with a toy trying to learn the feeling, then when masturbating doing kegels and reverse kegels without a toy to stimulate those same muscles. It has made masturbation more intense, my muscles aren't in good enough shape to do it continuous but when I do it the minutes leading to orgasm I have started ejactulating before I orgasm, and few big spurts, then the orgasm starts, so I think I'm on the right path. They are usually some serious ball draining ejaculations.
I have on little sub GF who is giving me tantric lingam massages every other week or so and working my prostate. I'm pretty sure within the next couple times she will get me off from just prostate stimulation. I've heard if you go right to cock stimulation you can have a multiple orgasm, so that will be next to try ;-)
Just need to find a few more partners who want to spend time between my legs