Schaff is quoted as follows:
"...the great majority of the changes of text...(probably more than 4/5) which they finally adopted had been anticipated by previous translators and commentators, and had become the common property of biblical scholars before the year 1870. But these improvements were scattered among many books, and lacked public recognition. They had literary worth, but no ecclesiastical authority. They were the work of individuals, not of the Church."
Schaff's understatement demands some remarks. Putting aside whether the radical, naive and sometimes absurd opinions of German critics etc. were really "improvements", what needs to be faced is the heterodox, fringe group nature of the motley crew that made up 19th century 'scholarship'. The Reformation had marched on by this time for 200 years, and spawned many 'denominations', sects, cults, and maverick loners.
The majority of the 19th century 'reformers' interested in revising the NT text were Unitarians, and could only be classed as 'mainstream' in the sense that they were part of rapidly expanding movement generally to abandon orthodox Christian doctrine and embrace rationalist historical-critical attitudes. Although these men were not consciously dishonest, they were certainly biased and largely motivated by their theological beliefs, one of them being their belief that the NT had been 'corrupted' by Trinitarians and Roman Catholics to an extent unjustifiable by the actual historical evidence.
But the lineage of the modern critical text is fairly easily traced, from the initial paranoid marginal notes of Wetstein who feared Latin/Romanist contamination, to the grim insistence on the "oldest MSS" by Tregelles, the last in the line of attempts predating Hort.
The Westcott/Hort branch of textual criticism goes:
(1707) Toinard (Roman Catholic Priest) - use 2 oldest MSS + Vulgate
(1720) Bentley (Cambridge Master) - no witness newer than 5th century
(1751) Wetstein (Arminian, Amsterdam) - prefer the older manuscripts
(1776) Harwood (Unitarian Presbyter.) - abandoned Textus Receptus
(1796) Griesbach (German Critic) - 'prefer the shorter reading'
(1842) Lachmann (German Critic) - 'recreate the 4th century text'
(1861) Tregelles (Quaker, non-conformist) - oldest MSS only
(1869) Tischendorf (Russian, Orthodox) - prefer Old Uncials
For a more detailed picture of all the previous scholars and editors, complete with articles on each, try our page here:
Early Critical Greek texts
All these critics and editors built on their predecessors and repeated the same naive mistakes in both methodology and MSS/reading preference. Their opinions and their texts are remarkably alike, and Hort's text is essentially the most extreme editing job of the lot (excluding Lachmann).
For an estimate of Schaff's work on the PA etc., try here:
Schaff on the PA