You might find some interesting reading in these two Audiogon forums regarding the same topic:
https://forum.audiogon.com/discussions/upgrading-form-pass-integrated-amplifier-to-a-better-one
https://forum.audiogon.com/discussions/which-integrated-amp-3
https://forum.audiogon.com/discussions/upgrading-form-pass-integrated-amplifier-to-a-better-one
https://forum.audiogon.com/discussions/which-integrated-amp-3