Does upgrading the clock always improve the sound?

It depends if there is a clock jitter issue to begin with and if the upgrade has less jitter than the existing setup.
Jitter is much less the evil dragon that requires slaying compared to fifteen years ago. Manufacturers are well aware of clock jitter issues these days.

Just a start John, not one-sided.

Since there seems to be more pro-clockers, he might as well weigh-in the small side of the fence first, then go from there.

I did that for my CEC and found what it really needed.

I have a Resolution Audio CD-55 which I think is very good. I don't know if upgrading the clock would give any benefit. Does anyone have any ideas on some obvious low cost tweaks or mods to this player?

Great Northern Sound will mod the Resolution Audio gear. You might want to do some reading on their site.