Here is an arm that you should look at. I am expecting mine next week and it will replace a rewired RB-300 w/Tecno Weight & VTA adjuster.
While I agree that the Audio Mod arms are very good, your Alphason arm, IMHO, is one of the best arms ever made. I owned a couple of the HR-100S MCS arm on different tables and always found them to be top performers.
In your setup, I might try a new cartridge first. Maybe a Lyra Dorian, Argo, Helikon or a Dynavector.
Wow, I don't think that the venerable Alphason is the weak link by any stretch of the imagination. I would look at almost everything before that arm. Actually the whole rig looks pretty well put together. How about some detail work, tube swapping, AC power delivery (dedicated line, outlets, power cords), room treatment, or just some nice records. If you are set on an equiptment upgrade, I would look first at the phono cartridge.
I have the gyrodec se with a rb300 with michell tecno weight and incognito wire and soundsmith smmc-1 cartridge. Sounds very good but as always looking for something better. I am considering the Trans-fi linear tracking arm. It look and sound very interesting at a somewhat reasonable price in this hobby. Google it and look at audiogon discussion forum. Doesn't address your question but provides something else to consider.
I started out with an Incognito rewired RB300 on my Gyrodec and moved to a used SME309. The SME is much more refined and quiet, but the RB300 had a certain beefy, solid quality that I can imagine some people preferring. I paid $1100 for the 309 and think it's one of the best analog purchases I ever made.
Note that the SME alignment method work best with cartridges with a 9.5mm mouting hole to stylus distance. I think this *should* work OK with the Benz, but I remember my Benz L2 tracking better on the RB300 (I "downgraded" to an AT33PTG, which I love, even though it's a little too compliant for the SME).