I have had the F8n for over 2 years (? - last year is hard to count), and the F8 before that. Gain (75dB) and EIN (-127dB) specs are the same (F8n vs F6) and I believe they are essentially the same preamps.
I only used the F8n briefly at home, i.e., where I have treatment and some control over sound, but any noise in my space heard in that was always ambient, and in actual live use, preamp noise is never anything you'd need to worry about. They are effectively silent IMO, but my ears are long past the point of hearing everything. Now, it *is* a field recorder and not targeted at home recording, overdubbing, and that sort of thing, so if that's what you want, I'd look elsewhere.
Never used the F6 but I'm a fan of Zoom recorders since the first H2 I got a decade ago.