Cellular heterogeneous networks (HetNets) with densely deployed small cells can effectively boost network capacity. The co-channel interference and the prominent energy consumption are two crucial issues in HetNets which need to be addressed. Taking the traffic variations into account, in this paper, the performances of HetNets with joint further-enhanced inter-cell interference coordination (FeICIC) and adaptive spectrum allocation (ASA) for downlink communications are analytically analyzed for the first time. An analytical framework to evaluate performance gains via a stochastic geometric approach for two-tier HetNets is proposed. The overall signal to interference plus noise ratio (SINR) coverage and energy efficiency (EE) are derived and validated by Monte Carlo simulations. Numerical results confirm the effectiveness of the joint FeICIC and ASA scheme in terms of improving the SINR coverage and EE. Especially, the gains increase significantly with the growth of the pico cell density. The results indicate that joint FeICIC and ASA is a promising scheme for the next generation cellular networks.