1) ... it's nearly impossible to eliminate sub-sonic resonances without using an effective subsonic filter (also called infrasonic filter). At some point around 1980, most manufacturers of phono playback electronics started addressing the subsonic/infrasonic issues. 2) Additionally, up until the mid-1980s most stereophonic integrated amps and receivers offered a user-selectable mono switch.
3) Final thought: although turntable-induced subsonic noise is not present in live performances of music... For some listeners, the sensation of subsonic information is perceived and felt in a pleasing and enjoyable way, even though that subsonic information was not present during the actual musical performance.
1) With loosely damped woofers, especially reflex designs, it is quite easy to see woofer 'pumping' resulting from records and thier players. The JBL L100/4311 was a prime example of this. Filtering removes
visible pumping, considerably. Some say that acoustic suspension designs were less affected by it, which makes sense.
How
audible is it to the listener, in their living room? Back in the day (as you and others have mentioned), the filter was not so much employed in order to conserve LF cone excursion, but rather to keep downstream electronics from attempting to amplify an unwanted record player garbage signal, thus 'robbing' the amplifier of power. Unwanted woofer cone movement was always a secondary consideration--at least as the explanations went.
2) Most contemporary preamplifiers (and stand-alone phono stages) are not well designed from an analog oriented ergonomic standpoint--I mean with the end-user in mind. Records/cartridges often have balance imperfections, which can only be adjusted using a balance control, and in the case of monophonic (and 'stereo reprocessed' records), an A+B switch is helpful. Also, some records have specific channel imperfections (scratches and pressing defects) that can be somewhat mitigated by the use of either an A channel or B channel selector. Of course with mono records, the best way is just to listen using one speaker, as nature intended.
You are correct pointing out how all this was common on most front ends until the early '80s. I have heard various explanations, including the 'minimalist gives better sound' version, but today I suspect that features are left out due to price considerations. Plus, our digital world doesn't usually require the switches, and if you need them there, they can be found in software.
In addition, the demise of analog consumer tape recording plus FM broadcasts have made these features less important to consumers.
The point is, even in quite expensive integrated amplifiers sporting a 'retro-trad' look, and that advertise top-tier phono involvement (such as Yamaha, Lux, and Accuphase), you won't find all the once common features.
3) Certainly-- but we know how in any live venue, other extraneous noises happen, noises one easily ignores, yet can be quite annoying once captured with a microphone. In a way, a 'too perfect' recording might well be perceived as more unnatural sounding than one containing low level noise artifacts (including added artifacts such as reverb and other types of studio signal processing). Generally, once the music starts, record player noise is masked depending upon loudness, so YMMV. And like extraneous concert noises (the guy two seats over eating a bag of potato chips, and the lady with a stuffy nose), I suspect that most record listeners just learn to ignore it, more or less.
