That is exactly what I'm running with no complaints (well, one complaint). The M3500 tape inputs are -10 unbalanced and the outputs of the HD24 are +4 balanced. They'll still play nice together and I think it sounds pretty damn good.
It's not so bad. Routing is pretty flexable and 24 tape returns plus 24 line inputs for aux returns (not to mention the 4 actual aux returns) is pretty handy. I don't have any appriciable noise in the system.
