'Supposedly, there exist a few 'triode' (tube) amplifier designs of exceptional transparency. Usually their power output is below 20W and insufficient to drive most speakers'... most important are low output impedance (<0.4 ohm) and low distortion (<0.1%) of the amplifier over a 5 Hz to 50 kHz frequency range' (Seigfried Linkwitz).
The best I could find among the popular tube brands is 1-2% THD, and only over 20hz-20khz range. 'Rogue' looked better than any of those. But I am not sure the compromise in transparency (low distoriton) is worth the 'bloom' of tubes when you need more than 20w to power your mid and upper range drivers. (always use ss for woofers)
The ATI line of solid state amps are <0.005THD, for instance. As are the vintage Hafler DH-200/220, the Muse 160 (and Bry$ston, and the very $$ Jeff Rowland) solid state amps.
If you just want a 'warm', 'soft' sound, put cotton in your ears, or hang a blanket over your speakers. It will accomplish the same thing. Really.