I feel your pain. Bass management is a thorny issue.
Many integrateds have line level pre-outs but they, as you probably know, contain a full-range signal.
Unless you're into surround sound receivers (which have bass management capabilities built-in), then I think your only recourse is an external crossover. There are good ones available .... the Paradigm X-30 gets good press around here.
Have you thought of seperate pre- and power amps? You could place a crossover between them.