If anyone is still looking for a SLP-98

You are right on both questions.
Yes, when the power switch is ON on the PS it is only applying power to the relay board inside the PS. This powers a small 12V transformer that powers a relay. The relay is then activated by the switch on the main Preamp when it is set to ON.
The switch on the preamp connects a ground wire back to the relay in the PS which turning on the power to the main transformer. That transformer provides the high voltage and the filament supply.
The reason for all this is to keep all AC voltages out of the actual Preamp chassis. The Preamp switch triggers the relay by connecting it to ground.
Am I explaining that well?

P.S.  The meters are connected to the DC voltage coming from the rectifier tube like you say.  They are not perfectly accurate but are a great reference point to keep an eye on.  Once warmed up they should always read pretty much the same.  Baring some AC voltage variations from the house wiring.  Sometimes people freak because the current is reading a little low.  You may also notice  then that the voltage is a little higher that day. 
Where the meters are useful is if you suddenly notice a higher current draw.  This would mean there is a short somewhere.  Start by changing the rectifier tube.  If a new tube doesn’t correct the high current draw then you either have a 6SN7 tube shorting or a bad PS capacitor.  If you pull out all the 6SN7 tubes and turn it on and it still drawing high current with a good rectifier tube then you know you have a shorted capacitor somewhere.  

In my setup I’ve found I like the old NOS Sylvania 6SN7 chrome top tubes except for the balanced input position tubes.  Because my DAC tends to be a bit lean for my taste I really like the Sylvania 6F8G tubes (which need to be used in an adapter).  These tubes sweeten things up nicely yet don’t loose much in the way of dynamics.

@immatthewj 

I didn't mean to hijack your thread but I guess I more or less did take it over. I should be more mindful in the future. Sorry. 

I didn't mean to hijack your thread but I guess I more or less did take it over. I should be more mindful in the future. Sorry. 

@billpete , that comment was not directed at you or anyone else!  If you took it that way, I sincerely apologize!  It was just kind of an offhand joke since I started the thread out as a heads up for anyone looking for a SLP-98 and then here I am asking Harpo SLP-05 questions.  Honestly, @billpete , threads evolve into other topics all the time here on A'gon, I have done it on many other threads by other people, and I honestly don't care what anybody types about on a thread that I have started.  Again, if you interpreted that as a snark I was directing at you, I aplogize because I did not intend it as a snark towards anyone, except maybe myself.  And after I post this reply, I am about to divert my thread some more!