To Float or not to Float...

 I also have a pair of DeHavilland mono block amps with the same ground/float switches, and in the manual it states that leaving the switch in the "float" position may result in a "cleaner sound".  At any rate, a fully functioning grounded power cord has done the trick...

If the grounding scheme of the audio equipment is properly designed and executed there will be no need for a ground lift switch. The chassis will simply be properly grounded through the AC power cord's third prong and doing so will in no way affect the sound.

@jea48 has been correct in his assertions.

linndec

... if you have a ground loop "buzzing" noise lift the earth on the pre amp as interconnects can more than likely handle that current going back to earth.

This is bad advice and potentially unsafe. Interconnects aren’t designed to handle this current, and lifting the AC ground could energize component chassis with lethal voltages.

It amazes me how often people here misunderstand ground loop problems. The issue isn’t having multiple paths to ground; the problem is having multiple ground paths at varying potentials.

This is one of the benefits of using dedicated AC lines - it simply makes it easier to have clean grounds. I have a biamplified system with lord knows about many ground paths! I don’t have any hum issues, though, because of these clean, direct grounds ... all at the same voltage potential.

If your equipment has a ground-lift switch then it should be safe to use if you desire. Whether the connected components have the earth ground prong or not doesn't matter as long as they're properly designed. Simple as that. 

Unless you plan on opening your gear while it's connected to power (or immediately after a disconnect) then you have nothing to worry about. 

 A strong surge can destroy your gear regardless, so get redundant surge protection on your lines. 

I corresponded with Kara Chaffee, the designer of my preamp and mono blocks, and she concurred with Jim's post of 9/16/2017. The "float" switch only lifts the audio circuit from the cassis ground and the AC mains equipment ground remains connected to the chassis through the 3 wire grounding type power cord. Kara feels the units sound best with the audio circuits floated and that's the way I've always run them with no noise or buzz (except for aforementioned power cord malfunction).