I have kept my ARC LS-1 all these years expressly for it’s mode switch---Stereo/Reverse/Mono/Left/Right. Before the current rash of 60’s albums issued in Mono on LP and CD (Dylan, The Beatles), the Stereo pressings of albums I wanted to hear in Mono (not having a Mono cartridge) would go into my main pre-amp, the Stereo signal sent out the tape out jacks to the LS-1, where the Mono switch would be engaged. The now Mono signal would be sent back to the main pre via the main output jacks of the LS-1 (the Mode switch is located after the tape output in the circuit) to be heard. Sure, the extra signal processing (extra pre-amp and two pair of ic cables) resulted in a loss of ultimate transparency, but those 60’s albums don’t have that great of sound anyway. I have always played my Mono LP’s with a Stereo cartridge, but a Mono cartridge is in my sights.
A Mode switch is also fun for listening to the Left and Right channels of 60's albums that have the instruments on one, the vocals and other overdubs (tambourine, etc.) on the other. Early Beatles, for instance. You can also switch between the two to hear the God-awful sound of "Electronically Reprocessed For Stereo" (Mono mixes with phase and frequency distortions added) 60's albums, offered for $1 more than the Mono version. In the late-60's, all the early-mid 60's Mono albums of The Beach Boys, The Kinks, The Yardbirds, etc. were cut-out of the label's catalogs, a hole was drilled in the cover or a corner of it cut off, and sold for 59 or 69 cents in drug stores, markets, etc. We collectors made the rounds of the stores regularly, looking for copies for either our collections or to sell/trade to/with other collectors. I still have hundreds of them in my LP racks.