It sounds like you love the marantz and would like to keep the grado statement. This is probably the best idea as the statement is pretty high up on the analog food chain and IMHO, going to be hard to top with a high output cartridge. There are better cartridges but all are low output moving coils. The grado reference 4.8mv version or the benz m2, while they have suffient output for the marantz 7, they are not the sonic equal of the statement.
I do have a solution. What you need is additional gain, and the way to accomplish this is with a stepup transformer.
Now, these can get very expensive, but there is a very nice one from EAR called the MC3 which is reasonably priced at $995. This is different from an outboard phono preamp as the stepup transformer doesn't have a RIAA network, which you don't need, as you already have the RIAA network in your existing phono section. You can get more info at the EAR USA web site as well as a list of dealers. If you do decide to go this route, this is one time I would strongly reccomend that you purchase it thru a dealer who will help you set it up and optimize/integrate it into your system.