  Measurements of the cosmic microwave background (CMB) temperature
anisotropies have revealed a dipolar asymmetry in power at the largest scales,
in apparent contradiction with the statistical isotropy of standard
cosmological models. The significance of the effect is not very high, and is
dependent on a posteriori choices. Nevertheless, a number of models have been
proposed that produce a scale-dependent asymmetry. We confront several such
models for a physical, position-space modulation with CMB temperature
observations. We find that, while some models that maintain the standard
isotropic power spectrum are allowed, others, such as those with modulated
tensor or uncorrelated isocurvature modes, can be ruled out on the basis of the
overproduction of isotropic power. This remains the case even when an extra
isocurvature mode fully anti-correlated with the adiabatic perturbations is
added to suppress power on large scales.
