Vertigo, this is a complicated subject and it varies a lot by specific record. The key factor is the mastering and pressing quality of the two records you're comparing.
One certainty in all of this is that you can tell nothing based on the record being sealed. The original releasing company made many many re-pressings of popular records and the quality from pressing to pressing can be all over the map. What matters is the matrix information inscribed in the deadwax of the record, and you can't get to that if the record is sealed. The matrix and other deadwax information will allow a knowledgeable person to tell something about the provenance of that record: who did the mastering, was it from an early stamper, is it a later reissue, etc. Even then, pressings vary by plant doing the pressing and from what stage in the life of a given stamper the particular LP came (was it within the first thousand LPs stamped, or the last five thousand). And for popular music recordings, very usually the earlier the stamper and mastering, the better the sound quality.
Many of the recent reissues have been simply superb and I've found myself preferring many of these more recent reissues to early originals (based on my listening priorities and what is important to me sonically, of course). The reissues will always sound at least *different* from an original. I can tell you that the Sundazed mono reissue of Dylan's "Highway 61" sounds great on my system. It's also virtually noise-free, something that will be hard to find looking for an original.
Sorry to be so general and non-specific, but I don't believe there is any simple general answer to your question. The answer is very much record-by-record specific.
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One certainty in all of this is that you can tell nothing based on the record being sealed. The original releasing company made many many re-pressings of popular records and the quality from pressing to pressing can be all over the map. What matters is the matrix information inscribed in the deadwax of the record, and you can't get to that if the record is sealed. The matrix and other deadwax information will allow a knowledgeable person to tell something about the provenance of that record: who did the mastering, was it from an early stamper, is it a later reissue, etc. Even then, pressings vary by plant doing the pressing and from what stage in the life of a given stamper the particular LP came (was it within the first thousand LPs stamped, or the last five thousand). And for popular music recordings, very usually the earlier the stamper and mastering, the better the sound quality.
Many of the recent reissues have been simply superb and I've found myself preferring many of these more recent reissues to early originals (based on my listening priorities and what is important to me sonically, of course). The reissues will always sound at least *different* from an original. I can tell you that the Sundazed mono reissue of Dylan's "Highway 61" sounds great on my system. It's also virtually noise-free, something that will be hard to find looking for an original.
Sorry to be so general and non-specific, but I don't believe there is any simple general answer to your question. The answer is very much record-by-record specific.
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