Four models were also produced in keyboard versions, all using the same hardware: the XL-6 Xtreme Keys, the MK-6 Mo' Phatt Keys, the PK-6 Proteus Keys, and the Halo which was marketed under the Ensoniq name. The Proteus 2500 was a 4U high rack version with a greatly expanded user interface for easier editing and real-time control. About 10 modules were released in this series, including the Audity 2000 (named after a legendary E-mu prototype that never went into production), Orbit 3, Planet Earth, Mo' Phatt, Xtreme Lead, and Vintage Pro. This series is distinguished externally by the five parameter entry / select knobs to the left of the display screen, easing somewhat the task of patch editing (the previous series all used a one-knob interface and had the reputation of being difficult to edit). The third series began with the introduction of the Proteus 2000 in 1998 originally but it developed in 1999. These corrected one of the main drawbacks of the first series, in that a VCF-like capability was available, providing more possibilities for altering the sound of the built-in samples. This line included the Orbit 9090 (electronica-centric samples), the Planet Phatt (hip-hop / acid jazz), the Carnaval (Latin sounds), and the Vintage Keys (samples of vintage electronic instruments). These units can be distinguished by a power switch at the far right with a large data entry knob just to its left. The hardware developed for the Morpheus in 1993 was used to develop a second series of sample playback synths. The sound shaping capabilities of these modules was limited, as they lacked any sort of filters. Some basic changes to the hardware design resulted in the 'XL' models which contained two or more of the ROM sets, and expanded patch memory. All four of these models shared the same hardware they were identical except for the sample ROM and panel graphics. This was followed shortly by the Proteus/2 in 1990, loaded with orchestral instrument sounds, the Proteus/3 in 1991, with ethnic instrument sounds, and the Procussion, with drum sounds. The sounds were basic rock and pop sounds - guitars, basses, (non-synth) keyboards, drums. It was loaded with samples in ROM, drawn from E-mu's library of sounds that it had built to sell to Emulator owners. The series began with the Proteus/1 in 1989.

Most were packaged as 1U high rackmount units, although a few models were built as full keyboards. There were about 20 models in all most were not named Proteus but they shared a basic architecture. A series of sample playback synths produced by E-mu Systems from 1959 through 2003.