^I think it may also depend on how you react to amphetamines too. I'm scripted dexamphetamine for adult ADD... so amphetamines probably effect me differently to most people. They still increase energy, motivation etc but in a more calming sort of way. Dose high enough though and regardless of any medical conditions and you'll be bouncing off the walls (if not physically, atleast mentally). Speed is speed at the end of the day.
I too am prescribed Dex for ADHD and have a lot of experience combining Dex / other amphetamines and LSD.
Your definitely right about mixing stims and psyches as 'playing with fire'. You really need to know your own mind, tolerances, how drugs effect you amongts lots of other factors. Probably not a good combo for most people. I never used to, but after using acid a bunch of times I tend to get a kick out of feeling almost insane and manic on it, which speed definitely adds to. I must be in a minority though for this. Crashing off amps + tripping = hell.
Add another to your minority list
I think there is a really good point made here, you need to know how you react to each substance individually before a combo can be assessed as a good idea or not and then you have to expect it to be different too.
I have found that people that tend to get increased anxiety from amphetamines are likely to have problems increased when combining LSD and amphetamines. I personally only notice an increase in anxiety (I'm pretty anxious normally) when coming down from amphetamines and find there use to be calming and reduce anxiety.
For me taking a recreational dose of amphetamines and then taking LSD is a lovely experience and one I have often. I have never personally had a problem from this combo.
I'm also one of the few that has never had a lasting bad experience (what most would call a bad trip) whilst on LSD though. I've been in some tricky situations before but I've always come through them feeling better for it.
I've had a bad experience on a combo of amphetamine and 2C-i but my experience with that combo is so limited I wouldn't like to draw any conclusions about it.
Bottom line is combining substances can lead to unexpected results and they are hard to predict. I don't advise it.
Doesn't mean this particular combo is bad for everyone or all the time. Personally I love it. I feel more psychologically open to the wonders of LSD when on amphetamines. I have some strange (uncommon) responses to drugs though and my understanding is most people are likely to not share my experience.
). Amazing body rushes. Later that day, I was fairly shattered after the LSD and speed wore off, so I took 30mg of temazepam which levelled me right out. Slept later that night. 
*hugs*