Have you thought about a Dada service/upgrade kit? I found It gave a bit more life to the music with my 306.
Replacing all the carbon film resistors for metal film (MRS25) In my 306 gave a bit more life to the highs, but without sounding harsh. Does the 909 have carbon film or metal?
Replacing the Input coupling cap (C2 on the 909) made the biggest Improvement In my 306. I used a HUGE Audyn true copper max. It was a tight squeeze to fit It, but well worth the effort.
I also made an air-core Inductor (L4 on the 909). Mine both measured 1.35u, so that's the value I replaced them with. 1.4mm enamelled copper wire, wound 11 times round an AA battery and carefully squeezed together with cable ties until they measured 1.35u. They also measure the same as the original ones In capacitance and resistance but without reverting back to the old ones I'm not sure If there was an Improvement.