I switched from an Evolve 40 to an Evolve 60 when I switched from double LP85s to double LP104s. I might have continued to get away with the Evolve 40, but with the heavier tanks it was right on the edge of not keeping my head above water.The Evolve 40 is nice for double AL80s and up to double HP100s -- really well designed for placement of the dump valve even if you have a drysuit inflation cylinder attached to your backplate and other small details like that. I was able to grab a used one for about $250 by being patient and watching the buy/sell groups on Facebook. It should get you through at least Tech 1 before you need more lift for multiple deco or stage bottles and larger back gas cylinders.