  Motivated by the current controversy over the redshift distribution and
physical properties of luminous (sub-)mm sources, we have undertaken a new
study of the brightest sample of unlensed (sub-)mm sources with pre-ALMA
interferometric follow-up in the COSMOS field. Exploiting the very latest
multi-frequency supporting data, we find that this sample displays a redshift
distribution indistinguishable from that of the lensed sources uncovered with
the South Pole Telescope (SPT), with z_median ~ 3.5. We also find that, over
the redshift range z ~ 2-6 the median stellar mass of the most luminous
(sub-)mm sources is M_* ~ 3 * 10^11 M_odot, yielding a typical specific
star-formation rate sSFR ~ 3 Gyr^-1. Consistent with recent ALMA and SMA
studies, we confirm that source blending is not a serious issue in the study of
luminous (sub-)mm sources uncovered by ground-based, single-dish surveys; only
~ 10-15% of bright (S_850 ~ 5 - 10 mJy) (sub-)mm sources arise from significant
(i.e. >20%) blends, and so our conclusions are largely unaffected by whether we
adopt the original single-dish mm/sub-mm flux-densities/positions, or the
interferometric data. Our results suggest that apparent disagreements over the
redshift distribution of (sub-)mm sources are a result of "down-sizing" in
dust-enshrouded star-formation, consistent with existing knowledge of the
star-formation histories of massive galaxies. They also indicate that extreme
star-forming galaxies at high redshift are, on average, subject to the same
star-formation rate-limiting processes as less luminous objects, and lie on the
"main sequence" of star-forming galaxies at z>3.
