NOW HEAR ME: I am NOT A Mastering Engineer.
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BUT: I have used the DW Fearn VT7, so I hope this qualifies the rest of my words for you.
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I would suggest that DW Fearn recording equipment is arguably found at the top of the LIST comprising the most WELL BUILT/MANUFACTURED TUBE equipment in the United States today. Everything he builds is remarkable and I find that it is all able to make poor sonic quality better, as far as my ears can define the word. The VT7 yields an extremely euphonic tone while representing the source and its depth. The overall sound of the VT7 is something I can only describe as "finished". Now, I am not a ME, but I am sure the job involves FINISHING.
I used to clean/detail automobiles in high end car and bike shops and I find audio gear and audio in itself, quite analogous to this process. I would machine buff/polish and then hand wax every panel/chrome on these things until the shine was blinding. Then I would make sure it stayed that way for as long was possible with as many coats of wax/sealant as it needs. Doug's boxes do this to your audio in one shot and you never need any wax to keep it around. Maybe to keep the charm from fading on the faceplate, though I think the charm will never fade from these animals.
The Gain reduction element/circuit uses a specially designed PWM [Pulse width modulation] board that is Solid State Technology. It has a similar "gain action" to a Crane Song STC-8, but because of the way it is laid out [more of an old school fashion] you don't need a rocket building certificate to use the thing. Its incredibly simple to operate and you do have a useful range of control for program. Its really a DREAM to TRACK with the VT7, but I think its a master buss/mastering dynamo, if the goal is enhancement of audio character, articulation, natural tone, and highly transparent squeeze that is radically in-audible over few-to moderate DB squeeze.
Its very capable and squeeze's quite usefully without artifact at heavier reductions. You can make it re-act, but it takes a while to get it to bear its action. Its a smoothing effect when used correctly, and really helps shape everything together while feeding audio amps that are beyond the term musical. The HPF link feature is CRUCIAL, and I think it was a worth while addition to the design.