Found this thread randomly and thought I'd throw my hat in the ring, months late or otherwise...
The "boosted gain" amp style is very modern, IMO. As far as I know it wasn't used for metal tones before the 90s, in the way it's now used. It is used for throwing a mid-boosted, low-cut signal into a high gain amp to give a tight, focused sound. It's not at all to get "more gain", in fact it excels in allowing you to use less gain, but still get a great metal tone.
The HBE is gainy but dry, and works well without a boost. It doesn't need the "tightening" effect because it's already very tight by design. It doesn't need the mid-boost because it isn't used with enough gain to really need that, generally speaking. Things like Rectos, on the other hand, work great with the mid-boost (and then mid-cut on the amp) and the low cut, because they can be flubby.
The main tip I have for 80s tone is to use a mid-boost before the amp, but not necessarily an overall signal boost. Boost 800-1000Hz into the amp, and give it quite a bit of juice. Don't boost the signal otherwise - in fact I use a Filter block for this, or a Drive with "Null" clipping selected.
You'll know it immediately when you hear it - that pushed 1k into the amp is the most 80s thing there is........ perhaps after dripping wet chorus!