Used preamp under $400...

As George stated, passives are uncolored, for the most part. When using the rear level controls on the 750, start out with them at minimum, and raise them gradually.

I may be selling my tube pre,, ~~$700 OBO in about 2-3 weeks. It can be seen/heard in my YT uploads, has  all new Mundorf Silver caps. 
Its a  real gem, clean solid 12AT7  tube sound, paid $700+ mods of $500. My loss your gain,,,that is if the shootout with the Jadis DPL which arrives tomorrow, loses out vs the DPL. If the Jadis loses, then it has to go, and keeping the 12AT7 pre. 
Just giving you heads up. 
ebay has a  12ax7  similar to mine for like,,,$500

I see others have beat me to the punch, but I'll say it anyway.  Schiit Audio Saga +.  You can also get a b stock version for a little less.  I've had the old gen Saga, which was my first passive.  It was so good I thought I'd try out the Freya +.  I'm not disappointed!  

The old Saga was better than my Parasound Halo P5.  They refined their pre's significantly so I expect that the new Saga + would be even more of a giant killer.

for $400 you ain;'t gonna get much.

hi the schiit saga+ is passive