It can be seen in this photo from the 1979 McCartney II recording sessions.

An excerpt from a interview by Dr Fink and Rosie Gaines “ What instruments were you playing then? Fink: By the time I joined him, my peronal setup was just a Minimoog, a Rhodes, a Farfisa VIP 345 with a Leslie speaker cabinet, and a Freeman string synthesizer. At that same time, he had all this gear he bought with money from his record deal, so I integrated what I had with what he had. I got rid of the Freeman and went to an ARP String Ensemble, and the old Oberheim Four-Voices and S.E.M. modules. I kept the Minimoog, and added a [Hohner] Clavinet and an ARP Pro-Soloist... a lot of stuff! It was a Rick Wakeman kind of setup. Then, as time went on, I pared things down, and every year kept updating as things changed”.

In this image, Vangelis can be seen at Nemo Studios surrounded with studio equipment, including an ARP Pro Soloist.

In this interview with Sound On Sound, it is states that Tony Banks used an ARP Pro Soloist.

In the Thumbnail of the video you can see it in his keyboard rig

Gary Numan is credited with ARP Pro Soloist in the liner notes for Telekon.

Key lists the ARP Pro Soloist as one of the synths he has used in an interview with Reverb.com

In 1980, Key would join the new wave band Images in Vogue as a drummer, gaining access to a Sequential Circuits Pro One and Prophet 5, an ARP Pro Soloist, a Moog Source, the Syncussion, and all sorts of drum machines.

In the source provided by The Concert Database, Jon Lord is seen with an ARP Pro Soloist synthesizer positioned directly behind him, indicating its use in his performances.

"As for hardware synths, Rjd2’s Clavia Nord Lead Electro, Korg Polysix, Yamaha CS-80 and SY-2, ARP Pro Soloist, and Moog Minimoog provide color to his beat-pulsing tunes. The “synth room” holds an ARP 2600, Moog Polymoog, Sequential Circuits Prophet 5 and Prophet T8, Rhodes Chroma, Oberheim Matrix 12 and Oberheim OB-Xa, Roland Juno-60, Elka Rhapsody 490 String Machine, and Elka Synthex."

"For this present tour, I play one large keyboard rack containing the PPG2, PPG1, Korg Polyphonic Ensemble (which I use only for strings), and the Arp Pro-Soloist. There is also a Conn Strobe Tuner for matching pitches. To my right I operate the Soundcraft Series 1S 20 into 4 mixer which takes both keyboard and guitar signals. For the guitar, which is a Gibson on this tour, I have an Ibanex UE400 Multi-Effects unit and an MXR Digital Delay. The Ibanex effects are controlled by a foot switch box with compressor/phaser/distortion/chorus/flanger and I set the guitar volume with a Boss pedal."

So Glass, who had a conventional classical music training before becoming influenced by Indian music and studying with Ravi Shankar, was quick to take up the opportunities afforded by electric keyboards (originally Farfisa organs and ARP Pro Soloist synths) on his earlier records such as Music in Similar Motion (Chatham Square), Music In Twelve Parts (Caroline) and North Star (Virgin). So why haven't other classical musicians followed his ideas of "extended instrumentation", overdubbing and multitrack recording?

In this interview, Bernie Worrell mentions using an ARP Pro Soloist as part of his rig.