Have you tried flipping the Figure 8 cord if it's a 2 prong type?
Try keeping the laptop power supply far away from the laptop, amp and audio cables
Otherwise you're stuck with 'dirty' SMPS style junk circulating at the mains power neutral to earth and into the audio return path earth etc
Well, something loopy nasty like that
You could buy/try a stereo or two mono audio isolation transformers and play with the earth ground switch/es on them
but keep the receipt and packaging with a firm OK to return them if still noise
FYI: audio transformers can differ in performance for various reasons including price,
and some are just plain wired wrong, they pass the audio great, including the offending hum and bonus harmonics and -BUZZzzz- !

so try a few out, even a cheapie car audio one might get the job done and sound ok with some EQ tweaks if required
Sometimes those laptop power supplies spew out interference that goes everywhere into everything,
usually something inside is cooking, died, dying, or a half baked suppression design, and you can't win
so battery power is the only option

Been there...
