If you are recording music with an average peak sound pressure in the 85db-95db range, and occasional transients hitting 100db+ you need to dial the sensitivity mic in all the way back to -18 on the deck, or you will get overload distortion. But if you are recording much softer music, or nature, you face noise from the internal preamp, so you need to bump the sensitivity just enough to get above the noise floor. While modified units with upgraded input capacitors and more robust opamps do lower the noise floor and increase headroom, it's still required to set the sensitivity right to get any benefit. A stock unit properly adjusted for mic input sensitivity will sound better than a modified 661 set up at the wrong sensitivity. One reason I like outboard mic pres is you can tape every show without having to go into the deck's settings menu and make changes.
Hope this helps