I actually didn't replace any transistors on the PM640 because they didn't have a complete set at Fry's.
So, I just replaced two original 56 ohm resistors with 1/4 watt 220 ohm resistors I got at Fry's. A pack of 4 cost 95 cents. After that the temp in the PM640 driver heat sinks dropped from about 69 degrees centigrade to 38-40 degrees centigrade.
I also have an HK 330i and it doesn't seem that hot at least not as bad as the original PM640 before replacements. The heat fins on my HK 330i measure about 29-33 degrees centigrade, and the heat sink on the single transistor is about 51-57 degrees centigrade at its hottest point.
But up to you if you want to get something better. I would try it while you have it though.
It's one of the sweetest, warmest sounding receivers I have and nice detail too.
The only shortcoming is it sounds better than it's pillbox looks. You may want to adjust the amp's idle current to optimize it. I think I noticed a more 3D richer sound.
http://manuals.harman.com/hk/Service Manual/hk330i sm.pdf
If you do some research, vintage HK can be a crapshoot. I passed on an HK 330B awhile back because of tuner distortion (not just static) during a demo. The seller later reflowed the solder and said it was fine, but I still passed based on a combination of price, cosmetic condition and didn't want to waste another trip.