  We have developed a Monte Carlo technique to test models for the true power
spectra of intermittently sampled lightcurves against the noisy, observed power
spectra, and produce a reliable estimate of the goodness of fit of the given
model. We apply this technique to constrain the broadband power spectra of a
sample of four Seyfert galaxies monitored by the Rossi X-ray Timing Explorer
(RXTE) over three years. We show that the power spectra of three of the AGN in
our sample (MCG-6-30-15, NGC5506 and NGC3516) flatten significantly towards low
frequencies, while the power spectrum of NGC5548 shows no evidence of
flattening. We fit two models for the flattening, a `knee' model, analogous to
the low-frequency break seen in the power spectra of BHXRBs in the low state
(where the power-spectral slope flattens to \alpha=0) and a `high-frequency
break' model (where the power-spectral slope flattens to \alpha=1), analogous
to the high-frequency break seen in the high and low-state power spectra of the
classic BHXRB Cyg X-1. Both models provide good fits to the power spectra of
all four AGN. For both models, the characteristic frequency for flattening is
significantly higher in MCG-6-30-15 than in NGC 3516 (by factor ~10) although
both sources have similar X-ray luminosities, suggesting that MCG-6-30-15 has a
lower black hole mass and is accreting at a higher rate than NGC 3516. Assuming
linear scaling of characteristic frequencies with black hole mass, the high
accretion rate implied for MCG-6-30-15 favours the high-frequency break model
for this source and further suggests that MCG-6-30-15 and possibly NGC 5506,
may be analogues of Cyg X-1 in the high state [ABRIDGED].
