It sounds like you are mixing the output signal with the H8000 input signal externally. Because there are slight delays between the H8000's inputs and outputs, if you mix a Thru' signal with its inputs you can get some top cut or comb filtering. You never actually need to do this, and with an actual effect it will not be an issue.
Where you cut gain depends on what you are trying to do:
1) If you have a very high level input which overloads the A/D you might need to cut the input gaintrim (pre A/D). This is uncommon because these inputs will take very high levels.
2) In the unlikely event that an effect overloads internally (usually only happens with extreme peaking EQ), you might want to drop the preset's input gain to fix this.
3) If the out put level from the H8000 overdrives whatever follows it, cut the "outputs/Post D/A Gain", rather than anything else – this will give the best signal quality.
