But of course Marshall typically didn't reduce the value of the 500pf bypass much or at all when they went to 470K (I know yours with the 330pf tan tube is an exception, Chris), so you typically get a little more perceived high end bite on that deal alone.
Another thing worth mentioning about the summing resistors and any bypass is that when the vol knob on that channel is at zero and you're using another channel, you've got that bypass cap on the other channel draining signal right to ground (in conjunction with the resistor itself). That's definitely one of the reasons that ChII on a split cathode Lead is often so stubbornly dull. Becomes especially pronounced in situations like a Super PA where you may have added bypass caps on several channels' mixers. One easy partial fix/experiment- turn the volume up some on the channel(s) you're not using. You'll get a little extra power and edge on the channel in use, along with more noise unfortunately. But on some older amps especially with less high end gain on the front end so noise is less of a problem this is like an added fine tune tone control.
Drewleslie- what I recommend initially if you want to hear the difference between 270K and 470K mixers holding everything else constant is not to swap the resistors but to connect with a jumper a 680K or 750K or 820K resistor in parallel with the installed 470K, so you can "convert" to about 270K instantaneously on and off for a real comparison to hear the difference. I wouldn't do it at extreme volume, but you can really tell what the difference is and isn't in the way you hear it that way, with a particular amp and tubes, etc.