Assuming you do have a tub drain, and not a clean-out as was suggested, and assuming you really donít want to mess with the concrete floor:
You could build a platform for the tub. It wouldnít be the first time I saw one. You might be making the tub too high to get in, or causing a headroom problem or code violation, etc. In that case you could build an entire sub-floor for the whole bathroom up about 6" to do what you need to do. Normally, this is done because the sewer line is higher than the basement floor, and the owner still wants a basement bathroom. The raised floor hides a pump system to pump the drain water up to the sewer lineís height. Again, it would be a question of headroom on whether or not it would be feasible in your situation.
Do you also have rough-ins for the sink and toilet? If you donít, I would guess you are not looking at a rough-in for a tub, but a clean-out, especially since most folks only put toilets and sinks in basements.
Last, you may want to consider a shorter tub, or even a shower stall.
Regardless, you will need at least enough room for a water trap so sewer gasses will not vent in the basement.