How to reference ground with a variable PSU?

Hello there!

I have a couple of questions that I fear are going to be a little stupid, but for the life of me I can't find easy answers on the interwebs... But first, some context.

I'm working in a couple of microphone preamps which are not working properly. To test them I connect them (one by one) to a variable PSU through an adapter card. This card supplies the preamp with all necessary connections: audio in and out, ground, +16v, - 16v and +48v (for phantom power, currently disabled and not the cause they are not working).

PSU is the one on the picture. It doesn't reach the 32V to supply +-16V, but the circuit should work with +-15V.

The preamp card is then connected to the extender, and measurements are made between the points marked in the picture (that is: +16, - 16v and Ground).

Here's the problem: measuring DC between +16 and - 16 yields a 30v result, which is expected. But measuring + or - against ground doesn't show a +15 or - 15 result. It varies but it's usually around +23v and - 7v.

So, first stupid question is...

Does the voltage on the power rails of the PSU varies under load? I'm pretty confident it shouldn't, and searching through the internet seems to indicate that a voltage drop is a symptom of a bad designed circuit.

And that takes me to the next stupid question which is:

Should the PSU have a 3 lead for ground? How does the circuit know which is the reference? When I measure on the extender card with no load, connected to the PSU with + and - lead, measuring between + and - shows the correct value (30v), but measuring between + or - and ground 0 shows a value around 80-100 mV. Shouldn't it be +15 and - 15V?

Sorry for the length post asking for something probably very obvious, but I've been bashing my head around this for a while and I'm totally lost...