Man.. these are just killin me

especially when it's like 7h9p7 or something with two fingers on the same string involved.

How do you do them so that when you let off the note rings just as/almost as loud as before you did it?
a bit of distortion/overdrive always helps. but you've just gotta listen to yourself doing and thinking "okay, whats the problem here and how can i fix it?". it may be something like the finger on the 9th fret is not pushing down hard enough, so neither hammer on nor the pull off sound full, or something else.

practice is all that is necessary.
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For the example you gave, make sure you hammer on nice and hard so that the 9th fret rings out as loud as possible. In doing that your finger should be in the best position for doing the pull off - and as you take it off, do so as if you're finger picking the string. You've got to really catch it; keep practicing.