It sounds like you are inexperienced in electronic measurement, and probably also in safety when dealing with high voltage equipment? If you do proceed on this, please do a google on safety. At a bare minimum, NEVER perform measurements with both hands. Keep one hand in your pocket or away from the equipment (and not touching metal, etc.) If it were me, I would use some clip leads of appropriate voltage rating and clip them onto the test points with the amp off/unplugged. Be aware that without bleeder resistors, capacitors can remain charged up to high levels, even with the equipment off and unplugged. They could stay that way for days without being properly discharged.
My advice would be to leave this to someone with experience.
Is this an amp that you just received, or have you owned this for some time? Not clear from your postings whether this is something that just occurred under your ownership, or if you bought it and it arrived working like this?