I have the pretty much the same rig, the only difference being I have a SDS and use an (solid-state) ASR Basis. I run all tubes except in the phono pre, so I know the absolute uncertainty of tube "is it necessary-ness". Sure, better tubes give you different sounds. Notice I didn't say better sounds. Save your money and wait with your phono pre until you know the company sound. Once you learn what you like and don't like about the sound, you'll be better able to deal with tubes.
Stick with finding a good cartridge. I use MM exclusively because they are cheaper--thus, you can buy more of them for the price of a single decent MC. Variety is the spice of life; that's particularly true of cartridges. (They are very similar to choosing tubes.) So many choices, so many voices. For that arm, you'll need something with a bit of heft and a medium to high compliance. Give heed to compliance over weight. At the moment, I'm using an Ortofon M20E Super, a light cart with a high compliance. I picked it up on Ebay for $149. It's a wonderful cart, very lively, very detailed. Tracks like a hound. One that fits your arm perfectly is the Nagaoka MP50. I like this cart a lot. It goes slightly higher, but overall, it's well-balanced. It's more expensive, but not MC expensive--$450, I think I paid. My favorite, however, is my Grace F9-E. Goes deep and high. Never shrill. Very smooth. Great on jazz, rock and even classical. Superior on vocals. It's not supposed to fit the JWM 9 arm, yet it works. Go figure. There are other, but these stand out to me.
I'm sure you'll get a lot of suggestions here. Use them. I did. Use the Vinyl Engine as well--it's a great source for information. Read through the Audiogon forums, there's a vast amounts of info there. Look at other members' rigs; see what they use. etc., etc., etc.
Yes, the Benz medium output is a good MC cart, one that works in your setup, one I'm thinking of getting, too--maybe.
Good luck. Have fun.
