I've never seen a 555 up close and personal but I have a 556 that I bought for 25 bucks with the full manual at a HAM/Antique radio fest. From what I know it's very similar to the 555 but has it's own power supply built in.
Other than a vertical driver tube and maybe 1 or 2 other tubes I've been running it as I bought it. It had a single and a dual trace plugin with it. I've since acquired 5 or 6 plugins from a flea market for about 10 bucks each. All had tubes and needed little more than cleanup. I then got a single bay (535?) with another 4 complete plugins.
Checked out the plugins but not the scope itself yet.
Fortunately I got to all those plugins before any audiophiles did to strip the tube out of them. ( I'm actually a tube audio nut myself but would never strip a Tek unit for an audio project!!!)
I built a cart for the 556 and it sits next to my bench always ready for action.
It too has quite an inrush when turned on. I have an old analog TV in my office and before I did some rewiring I could watch the picture get narrower and lights dim when I switch on the 556!
It used to be a "Hey watch this" moment when I had friends over:)
I love using it. Though it's only rated for 50 MHz I've easily locked on 100MHz+ signals (heavily attenuated.)
The internal construction of those scopes is a work of art to look at in my opinion.
I've not done a calibration on anything yet but with the plugins I use most form the basic checks I've done things seem to be pretty close to nut on. I'm sure it's been decades since a calibration was done on anything and possibly much not touched since it left the factory.
BTW while I'm here I do have a 1L5 spectrum analyzer plugin that I bought (cost more than the 2 scopes and all the other plugins put together) I don't believe it is working. I've checked the FET in it and did find 1 lytic that was bad but it appears to still have some problems. I need to make an extension cable to check it further and then that may become a discussion for another time here other than to ask if there is any common things to look for that goes bad on those?