Solodallas?

Here ya go. The Schaeffer wireless unit was used by many pros including Angus. It imparted some midrange boost/shift and compression that many loved. The wireless frequencies eventually were obsolete and the unit mfr was stopped. An AC/DC acolyte got an original and reverse engineered the audio circuitry, and sells a big replica tower, which Angus now uses apparently. He also made two pedal copies. The "Storm" is the latest and uses 9V. Here's a demo of the Storm. Pete Thorn also has a video out that tests the earlier pedal. Many videos out there.

Apparently subtle, but many say when you turn it off, it's missed.

The originals were really great and were used by many pros. They started remaking these a year or so ago in a pedal format and it made the price point a lot more affordable. The original components were hard to find but I hear that they they made a really good remake without much compromise and sound fairly true to the original. These added some light distortion with a treble boost but allowed for good note separation without getting blurred. Angus used one and so did Jimmy Page and many other artists and were kept off stage. It was the secret ingredient to many of those magical sounds that we just couldn't emulate without it. I watched an earlier video last year of the making and comparisons to the original.
I think the price was around $1200 or so. Don't quote me on this but the unit is just magical!!
If I had the chance to grab one, hell ya, I'd get it!!!

Darn. Didn't get it. Oh well, maybe later. Bought a Fender Santa Ana OD pedal instead for the same price.