What is the size/volume of your room? Given the high efficiency of the M60s and VP150, you are likely driving them at an average power of well below 1W, unless you listen at an insane level of volume and/or the room is very large. With the H/K 525 (70W/ch), it is likely that you already have nearly 20dB of head room. A power amp upgrade may or may not result in audible improvement, and I tend to bet it would not. If I were to go for it, I would aim at 6dB of power increase at the very least, which means 250-300W/ch minimum. I do not think anything less than that would worth it.
