Hardwood floor - how to avoid different levels between rooms
Was the plywood and 1x6 both on top of the subfloor?  If this is the case and the subfloor is solid and at least 3/4" I don't see why not. Just make sure to resecure and replace any damaged areas cause this can cause squeaking and affect the performance of the finished product. Remember to let the wood acclimate 4 to 7 days before installing. Use a moisture barrier and leave the proper expansion gap around perimeter and fixed objects.