Nothing wrong with the new AMP reg's, I don't see any improvement with HUMA, although I do use HUMA, whatever I have available. If I had to choose one it would be AMP.
The HUMA EHP reg only takes 8 washers 0.6mm thick. Not sure why they send 9 with them. I guess just to make sure you have enough. Cheaper than mailing just one I guess.
The hole is probably the atmospheric vent. At least that what the block covers. Id remove the block and replace all the reg orings and see if that solves your problem. Only then would I tackle the oring in the block. They are hard to see and fun to replace.
Here's my tip. When replacing the block oring, put just the reg piston and washers in the gun. The washers will keep the oring from going too far below the groove and make it much easier to get in. Don't let this oring go for years and years and get tough & hard, that makes the terrible to get out. If they're soft, as they should be, you can punch them with a hat pin and lever them right out. If they get hard, you have to keep picking at them until you break them or can get behind them and it's a job. My last one got really hard, took 2 hours to dig out and it broke to pieces in my hand, whew!