That link is what info I have. Changing the live weight from 20# to 40# only knocks off approximately 3' off of a 2x6 span. Take the dry weight of the tub, add the weight of 2 people, add the weight of the water capacity in gallons times about 8 to get water weight, and total the weights. Divide the total by the square footage of the tub to get live loading per sq.ft. I get about 67 pounds per sq.ft on a 5'x 32" tub with 50 gallons, 2 people x 200# and 100# dry weight. A 6' tub is close or less since there is more square footage to its base. Sounds like sistering the joists should do it which shouldn't be too expensive for an 8' span. For overkill, add a double sister to each one so you have a nominal 6"x6" joist. Use a mortar base under the tub.