audio / programs that were DSP-aware. Digitrax, maybe?

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Digitrax, yes.

http://www.soundonsound.com/sos/1995_articles/may95/applenotes.html

the latest acquisition by Opcode: DigiTrax from Alaska Software, founded by the Apple engineers who developed the core of the Apple Real-Time Architecture (ARTA) signal processing engine. DigiTrax 1.2 is a multitrack digital audio recording and editing package

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Also:

/not all checked: will add more as found/

Opcode Studio Vision Pro

Opcode Max with DSP objects from CNMAT also mentioned here.

Digidesign ProTools (various versions, mostly locked to their own hardware)

Digidesign TurboSynth - and possibly Antares Infinity?

Deck II (as mentioned upthread)

UniSon (same thread as Max)

Syd (formerly "SoundSynth")