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If he had 'changed them recently', knowing the stretches that many people who sell their amps consider 'recently' that could be over a year.
I changed my (bought brand new) roadster's preamp tubes after 2 years and only when I did did I notice how noisy and crackly it had been and how crap it was sounding.
I'd just buy a whole new set of preamp tubes if they're that kind of age.
Turn the amp up like you said and watch the power tubes, what you're hearing sounds like what you would if one of your tubes was arcing. You get arcing in a tube when there's air in there as the seals aren't perfect and air gets in over time. Air also means your filaments will be corroding with every use and a tube could go mid use which won't do anything any favours.
Be safe, consider a full retube, see a tech if you need reassurance.
most likely a bad/dying tube. since the previous owner said 'recently retubed', i'd estimate that your tubes are at least 2,5 - 3 years old. in this case you should be retubing anyway.
even if it's just a microphonic preamp tube causing the noise, i'd still retube the whole amp.