It could be that the old pads on the SR80 are somehow making the treble less harsh than the new pads on the SR60. Have you tried swapping pads between the two?

This one is correct, the Sony WH-1000XM4 look very similar but have different buttons and earpads

The headphones just have a different sound signature, the M30x has more bass than the T50RP. Getting an amp wouldn’t change how the headphones sound, just make them louder. If you think the bass on the T50RP is lacking, you have two options: either use EQ or buy different headphones.