If you are lucky enough to find a Vincent SA-T8 then this will give you what you are after. Massive step up over an AR SP-6 & AI L3. Fully balanced all tube design, has massive soundstage depth & width & oozes musicality in spades. Just has that "just right" midrange "warmth" and has none of the dry, etched, overly analytical presentation that plagues many Pre-Amps, which you are trying to avoid.
Replace the "crap" stock tubes with Psvane's and you will have a Pre that competes with or betters many Pre's costing multiples of it's cost.
Hope you can find one.