  Kilohertz quasi-periodic oscillations or kHz QPOs occur on the orbital
timescale of the inner accretion flow and may carry signatures of the physics
of strong gravity (c$^{2}$ ~ GM/R) and possibly clues to constraining the
neutron star equation of state (EOS). Both the timing behavior of kHz QPOs and
the time-averaged spectra of these systems have been studied extensively, yet
no model completely describes all the properties of kHz QPOs. Here, we present
a systematic study of spectral-timing products of kHz QPOs from low-mass X-ray
binary systems using archival Rossi X-ray Timing Explorer/Proportional Counter
Array data. For the lower kHz QPOs in fourteen objects and the upper kHz QPOs
in six, we were able to obtain correlated time-lags as a function of QPO
frequency and energy, as well as energy-dependent covariance spectra and
intrinsic coherence. For the lower kHz QPOs, we find a monotonic decrease in
lags with increasing energy, rising covariance to ~12 keV, and near unity
coherence at all energies. For the upper kHz QPOs, we find near zero lags,
rising covariance to ~12 keV, and less well-constrained coherence at all
energies. These results suggest that while kHz QPOs are likely produced by
similar mechanisms across the population of LMXBs, the lower kHz QPOs are
likely produced by a different mechanism than upper kHz QPOs
