With a reference to an upgrade in resolution and soundstaging to an existing quality system, and assuming the speakers are end game, the amp has proven compatible with the speakers, the pre compatible to the amp, and the source components are commensurate with the upgrade, I would upgrade the preamp over the amplifier.
Based on some comments that have been posted on other threads I hope my use of commas in that long winded sentence passes muster :-).
Based on some comments that have been posted on other threads I hope my use of commas in that long winded sentence passes muster :-).