Advice on Tonearm/ Cartridge combo

I've recently started using a SME 309 Arm on my SME rig, which is pretty low mass arm (9.5g).  The only cartridge I use and have is a Koetsu Black Goldline, which is a very low compliance cartridge around  (5 X 10-6cm).  I've been trying to make this combo work but these two just don't work well together, there's some resonance and distortion, especially with heavy piano chords.  Obviously I have change either the tonearm or cartridge.  Does it make more sense to get rid of the low compliance cart and get a more suitable cart to go with the SME arm, or should I trade in the arm for a heavier one that is more compatible with more carts.   

I had issues with Grado's Reference cartridge with my SME 309. Every time I would play one of those 45 rpm LPs, the first minute or two were unlistenable. I replaced the Grado with an Ortofon Cadenza Bronze and it was much better. Now I'm using a Lyra Kleos and love the combination. No distortion with either the Cadenza or the Kleos.
What were you using with that Koetsu, before the SME?     Were you enthused, with the Koetsu’s presentation, before the SME?      I could be wrong, but- it appears to me, that there are more good(as in, "upper tier"), higher mass, arms out there, than high compliance cartridges, from which to choose(though the aforementioned Kleos should be a nice match).      It’s also possible, that a used arm would be easier to market, than a used cartridge.     Does anyone offer a method/kit, to up the effective mass of that arm?
With such a light mass you definitely need a high compliance MM/MI cartridge, try some decent vintage cartridges and i'm pretty sure you will be impressed. Look for the best possible stylus profile and a nice cantilever, normally top quality MM/MI from the 80's are very nice. 

Or change your tonearm to something like FR-64s (very high mass) for low compliance cartridge. 
Koetsu Black Goldline is superb on my Origin Live Conqueror arm. Absolutely no problems with anything, anywhere, ever. Before the Koetsu, it worked wonderfully with a Benz Ruby. 

FR64S is a great suggestion. Also the Audiocreative Groovemaster II for a new high mass arm.