Reasonable ripple value for the tube amp switching power supply

Depends on many factors. For instance the post-PSU filtering, the signal levels in the amp (is it a preamp? microphone preamp? just a power amp?)

If the ripple is ultrasonic it might not matter anyway (and is easily filtered).

I've used this SMPS for instance with just an extra 10nF filter capacitor on the amp itself (yes, 10 nanofarad, not microfarad!):
DC12V to DC 200-450V 70W High Voltage Boost Converter Step Up Power Supply Board | eBay

Note that its a 12V input DC-DC converter, not a mains PSU, so hum wasn't an issue - for a mains SMPS you want one that's earthed, not a two-wire one.