Ah, I could go on ad nauseum about the equipment used in the 50's. I'll try not to do that.Actually, there were some pretty sophisticated EQ devices even that long ago. By EQ, I am assuming you mean tone-shaping. Let's talk about "real EQ" for a moment. The necessity to apply EQ came about as a result of the limitations of transcribing to vinyl. Groove velocities had to be limited and it was useful to reduce the noise floor as well. So, low-frequencies were cut and highs boosted into the cutter head with the inverse being applied at playback. In the early 1950's, before the RIAA curve became standard, there were no less than four different curves used (e.g., Columbia, RCA-AES, FFRR, EUR). Audiophile equipment of the day actually allowed the user to select among the curves. Despite (or because of) all of this, LPs were characterized by a high-frequency roll-off that is often associated with the "vinyl sound." Then we have the amplifiers of the day. Tubes, followed by output transformers, of course. The interactions of those components with the loudspeakers of the day all added up to a common lower-midrange rise resulting in that "warm" sound. It didn't hurt either that the vast majority of the music of the day (and right through to the 1970s) was mixed on the venerable British Invalid Link Removed(aka "coax') monitor. That speaker had the classic "British sound" which was-- you guessed it, lower mid-range warmth. Now, you take all of this and combine it with what others have mentioned about double-bass strings and playing style and you've got "the sound." (Okay, I tried to keep it short.
You did fine at keeping it short.
But remember that the EQ curves used to press and play back vinyl weren't that complex - more important, weren't user adjustable. What the engineers had available were generally simple shelving filters; even a 3 band EQ (with adjustable mids) were rare or non-existent in the studios of the 1950's. Pultec's revolutionary (for their time) EQP-1A was introduced in 1951, but they weren't in common use for recording individual instruments (many studios simply couldn't afford them) And while the Pultecs could boost (or cut) specific frequency bands, those bands were fixed. There were also issues that the higher a given frequency was boosted, the narrower the frequency range affected. Massenburg didn't invent (or begin to make) a paramatetric EQ that solved this problem until 1972.
While Tannoy may have been the speaker of choice in the audiophile market, more common in studios in that era were Altec Lansing cabinets - while the Altec 604 (introduced in 1944) remained the standard in the US for years, other Altec speakers were common. in fact Nashville's legendary Studio B had 3 A-7 Voice of the Theater cabinets mounted upside down in front of the console. The Tannoys became the standard in Europe, however.
And remember, when a mixing speaker has a uneven frequency response, mixers are prone to mix to the speaker, meaning that when those mixes were played back on other speakers without the mid-range boost, there would typically be a hole in those frequencies (an issue that still plagues mixing engineers today, making sure that mixes that sound great in the studio translate to the outside world).
I love this stuff - I've spent many, many hours talking to (and learning from recording engineers who were making records in the 1950's and 1960's; They tell great stories.
I have to go back to the idea that the sound was in the hands of the musicians and in their instruments. The room would have had an impact (large rooms were much more common then than today), and beyond that, in the skill of the recording engineers in knowing WHERE to put the right mic.
