Stereophonic two-channel reproduction became widespread after the two-channel record format arrived.

Evidence

The most common loudspeaker set-up is the two-channel stereophonic set-up. Its use became widespread after the development of the single-groove 45◦/45◦two-channel record in the late 1950s. Two loudspeakers are positioned in front of the listener 60◦apart, as illustrated in Figure 14.2a. The set-up enables the positioning of virtual sources between the loudspeakers, and it also makes the timbral quality of reproduction better when compared to monophonic reproduction, as discussed later in Section 14.4.1. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1805