CD v.s LP - When comming from the same MASTER

Sean: I'm not arguing that CDs can theoretically reproduce sound perfectly. They can't. I'm arguing that, practically, they come close enough. Yes, there is jitter, but all the research I've seen suggests that the level of jitter in a typical (and I do not mean high-end) CD player is an order of magnitude below hearing thresholds. DACs and anti-aliasing filters have improved since the early days, and I haven't heard or heard of a bad one in the last 15 years--except at the high end, where occasionally a designer is too busy being innovative to get the basics right.

The proof of this is in the listening. Remember that this thread started from the premise of a digital master tape. I would argue that you would have to listen very hard to discern a real difference between that master tape and a CD played on a $100 DVD player. Whereas it would be relatively easy to discern a difference between master tape and a $10,000 vinyl rig. That tells me that the implementation of digital, while still not perfect, is pretty darn good.

And, to get back to the original point, that means it's probably not the flaws in digital, such as they are, that are responsible for the fact that many listeners prefer the vinyl version. It's more likely that the technical flaws of vinyl are, counterintuitively, part of what makes that medium sonically appealing.

I don't quite follow the point about the necessity of summing deep bass to 'fit it on a record'. A full wavelength at 20Hz on a 33 1/3 RPM record traces 10 degrees. Put another way, you can fit 36 wavelengths of a 20Hz signal around a groove. Together with RIAA deemphasis, ~20db at 20Hz, this wouldn't seem to be a problem. What I don't know is what physical groove modulation is needed for a 0db level at 20Hz. Perhaps you're right.

At what frequency do they start summing to mono for LPs?

This of course completely begs the question of the necessity for stereo bass that low, unless of course your listening room is the size of a football stadium.

I think, it can't be answered in general.
The Problem with CD has nothing to do with the digital Master in general, most differences are made through the next processes. High speed burned or low speed burned, with or without reclocked burners and so on. It is more or less endless ( check out all these "remastered" issues, sick ).
Some CD's sound very good, some not and I think, it is the same with Vinyl, which has it's source from a Digital Master. Some CAN sound very good and some don't, depends on the Mastering Lab I guess.
Anyway, here I go for the CD, when I am not sure about the result, then I don't waste time on it.
( You know, I can freeze it, or paint the outer ring with a text marker or I can cut the outside for a better angle or ..... )

Pabelson and others...Summing of the bass (like the drastic RIAA equalization) is a topic that vinyl fans like to ignore. I believe that the reason for suming is to enable non-audiophile phono pickups to stay in the groove. Large vertical groove modulation would raise havoc with VTF, and even cause the stylus to hop out of the groove. I think that the summing is rather gradual in slope, but begins around 200 Hz. Whether this is a problem or not is another thread. Many people sum the bass to a single subwoofer. (Not me).

Horizontal groove modulation is what needs to be limited so as to save space on the vinyl. This is done, first by the RIAA equalization, and secondly by variable groove spacing (according to the momentary LF signal content).

An afterthought...Some of the very first stereo LPs, Audio Fidelity "Dukes of Dixieland" did not sum the bass, at least that I can detect on my system. Some of the very first stereo recordings (Vanguard in particular) also stuck with two microphones, and that also was better than early attempts at multitrack recording. Sometimes progress goes the wrong way.