Other worthwhile reading on the topic can be found in writings and accounts of public lectures given by participants of the Challenger Expedition. Unfortunately, it’s hard to find the references. Basically, the Challenger Expedition concluded that deep fauna is highly diverse. So, when and how did the general view that the diversity is low enter the discussion?

Murray, J. 1985. A summary of the scientific results obtained at the soundings, dredging, and trawling stations of the HMS Challenger. Challenger Report, Summary of Research 2:797-1608. Murray is quite specific in stating at while numbers decrease with depth, the variety of taxa increases and exceeds that found in shallow trawl samples.

Note, this is not one of the Report Series. It is a separate 2 volume publication.
The Voyage of H.M.S. “Challenger.” A Summary of the Scientific Results. (With Appendices). Two Parts. (London: Eyre and Spottiswoode, 1895.)

Potentially easier to find if you can locate old volumes of Nature are:

Moseley H.N. 1880 had pretty much already said the same in a series of lectures. Nature 21 (545) :p543-549, 21 (546) p569-573, 21 (547) p591-593.

The same for Jeffreys, J.G. 1881. Deep-sea exploration. Nature 23, (587) p23:300-301, 23 (588) p324-326.