The MS9 is my favorite chassis, and the tube generally tied to it (Toshiba A68KJU96X) is a fantastic tube. The same tube can be found in the MS8-29, Toshiba D29CR55, and some Hitachi GMK-xxx chassis. I have seen a Sanwa Z21C variant on the same tube too.
The tube has a lot to do with the quality of the image, especially for focus. The better focused the beam, the more precisely drawn the lines are. The A68KJU96X is a great tube, in my experience.
The MS9 has a nice little buffer/amp right near the inputs, and I have found that this amp has performed really well for my 15k sources, as far as not producing ringing artifacts and preserving sharp edges. The MS-2930 has the same amplifier, and there is a rare MS8-29 variant called MS8-29FN22 that has the RGB section from the MS9 on a daughtercard (and subsequently has a remote board that more resembles the MS9). This is the only MS8-29 I'd consider keeping.
Over the MS8, the MS9 adds a unity Contrast adjustment, which controls gain for all three channels. In addition, the gain controls are centered such that they don't move the bias / black level; in comparison, on the MS8, raising gain lowers effective bias for a channel, so changing the total display intensity while maintaining a consistent greyscale is a chore.