What I look at:
1] Power supplies: B+, bias & filament: Either they can be improved or there are issues
2] Gain stages, phase splitters: Usually cause hiss or pick up noise from other areas if not designed properly.
3] Wiring layout: Some times wire routing causes issues. With PC boards...you're stuck with what you have. Point to point wiring can be modified/improved.
My latest preamps are as quiet as battery operated ones. They don't hiss either. The newest NS-60 (enhanced version) is totally quiet, as in high end solid-state amp quiet. The design improvements implemented in them can in many cases be added to other gear.
There are ways to quiet down single ended amps as well.