Which would you choose for good sounding tube preamp for an audiophile new to tubes.

I need your help guys. I would like to add a tube preamp to pair with my reference 200.2 B&K amplifier. My speakers are large ADS L1590 towers ( 90 DB efficiency rating). They were a flagship back in the day. I have had them restored by an ads tech. I am l looking at  a QUICKSILVER LINESTAGE LS PREAMPLIFIER, VTL 5.5 preamplifier, and a Sonic Frontiers SFL-1. All are currently used and on audiomart or audiogon. The Sonic Frontiers is for sale on hi-fi heaven. I purchased from them before and they check out the equipment that is preowned. My system is a little bright and dynamic sounding using the sonata B&K preamp. (There best at the time). I am looking for a little warmth in the sound and no harshness. I want some slam and afraid I will lose it if I go with tubes.  I have had success buying certain units because of the knowledge dispensed through this forum I am a member of Audiogon in good standing. Any help would be great. I appreciate the feedback.