Experience on replacing capacitors of SS preamp?

Depends. What is your purpose for replacing them? Is there an issue with the preamp or are you wanting improved sound? 

Avoid changing caps in the signal path (shouldn't be any).
Find a 10K resistor or so to help you discharge caps. They can hold those very high voltages for a long time. Measure V before touching them. Use a resistor to discharge safely.

Ensure replacement caps:
V = or > originalTemperature = or greateruF = identical.

The uF in the primary filter caps can be a little larger (uF) but don't go nuts. You stress the transformer and bridge rectifier.

Higher Temperature = longer life.

Triple check polarity on removal and installation.

No, you only want to replace electrolytic caps in the power supply. Changing the film caps is not necessary and can affect sound and retail value.

I'm not sure if you are aware, but the volts in tube gear is lethal. Like 300 V or more is quite common.

So, pay extra attention to finding the voltage supplies, and measuring that they have fully discharged, using a resistor to drain high voltages.

So in the schematic, only worry about changing caps with a polarity (+) sign.

First, the preamp is old, second there is an audible hum even at the lowest volume.
Also, I’ve read many glorious stories of old preamps sounding much better after recaps, so I want to give a try by myself.
There are only 4 caps with + signs, so replacing 4 would be relatively simple, I hope.
Is Nichicon a good brand?
Any other recommendations for hifi audio grade capacitors?
thx

Audio grade is not usually what you want in a power supply. :)

But Nichicon makes excellent caps. 4 sounds about right.

Oops, I forgot you said it was SS. Typical volts in SS gear is 30 or less, you should be fine.