Can I get more out of my Koetsu ?

For what it’s worth, I am using a custom built, solid state current driven head amp in front of my tube phono stage with MM levels of gain. The results are impeccable, terrific. I love it. I did it because you have to go out of your way for gain if you are using an Ortofon MC 2000 cartridge, which has possibly the lowest voltage output of any cartridge ever marketed but produces decent current output owing to its very low internal resistance (2 ohms). And I don’t own a SUT. this type of head amp is real competition for any step up transformer.

Kudos to Sutherland for going commercial with the idea, if that’s what they’ve done. Because I can’t find such a product on their website.

I’ve seen various numbers for the internal resistance of these wood bodied Koetsus, but never so low as 3 ohms, except maybe for the Platinum version, e.g., Rosewood Platinum. My info suggests 5-6 ohms, but would still mate well with a current drive type phono. However, I don’t think such an expenditure is necessary. Also, my 12 year experience with my Urushi suggests that it is not just for the resonant frequency calculation that Koetsu likes high effective mass tonearms. The Urushi sounds best at all frequencies, bottom to top, with higher mass than necessary to bring Fr into the acceptable range of 8 to 12 Hz.

You HAVE already added mass to the tonearm, by using an 18g headshell.  So there is absolutely no need to butcher your turntable in the process of changing the tonearm. That said, my experience with an Urushi does suggest that Koetsus like a high effective mass.  Apart from messing around with your downstream equipment, you might try adding about 5g more effective mass to the arm, by simply sticking a weight to the top of your headshell.  Since the headshell and anything you stick to it are positioned directly over the stylus/cantilever, you can think of any mass there as adding directly to tonearm effective mass.  Furthermore, the necessary increase in mass of the counterweight also adds to effective mass; by a factor equal to the distance from the pivot to the center of mass of the CW-squared multiplied by the mass of the CW.  A US nickel or dime (I forget which) weighs about 5g.  Try sticking one to the top of the headshell using double-sided tape. If that does not improve things, then I would assume you should look elsewhere for upgrades.  And finally, consider that your Rosewood is already sounding as good as it can sound.  There's always that possibility.