A proof sheet shouldn't require any more light than any of the negatives would if they were in the enlarger, with the enlarger at the same height used for the proof sheet. That is, put an empty carrier in, adjust the height to produce an 8x10, focus the lens using the edge of the carrier. In this configuration, a proper exposure for the proof sheet will give you the proper exposure for an 8x10, or at least it should be very close.
If you mostly do smaller prints, then you'd need to reduce the light some for the enlargement.
As for the original problem, 1 1/2 minutes of exposure should be plenty, unless the enlarger produces very little light. I agree that a good first step would be a developer test using a completely fogged test sheet. If that works, then it would be helpful to know what enlarger, lens, etc. Especially if it has a color head. For example, in the case of a color head, what filtration is set, if any?