@BumbleBoy it's an absolute epic sex combo - cannot be beaten imo (my experiences though were with meth/ghb, so there's that)
HOWEVER - it could help to ruin sober-sex for you for a long time be warned
and - it can be dangerous too.
Best advice is get nice and high on the speed for quite a while then start adding the ghb. I cannot stress this enough - KEEP A DOSE RECORD OF THE G - edit - 2 hours between doses preferably)
You will not 'feel' the G as much (or so it will seem) even so, NEVER go more than 50% more than your usual G dose, and only do that twice before returning to your usual dose.
I would also suggest no more than 4 separate G doses (3 ideally), and make SURE that your amphet will still be in full swing when you're dosing. Dose too high when your speed wears off then you're falling out,
Don;t redose the speed to keep going, have your fun and then call it a night. No alcohol / downers.
Maybe a benzo, but at least 3 hours after your last G dose
Sounds like a strict regime but after what a mate of mine suffered, and what I came close to suffering - caution here is imperative
Don't injure yourself! (I gave myself a groin strain on one of my meth/G combo nights (only did two - would kill me now I fear!) and my partner was left high and, er, dry while I rolled around in agony for a couple of hours
Enjoy!