If you want to start with your existing power amps and start by adding a tube preamp these are the less expensive models that interest me, and I've been using tube gear (predominantly) since the mid 70's.
I'm out of touch with auditioning recent models and my interest in these items is based solely upon numerous online reviews (including those by A'Gon members who's opinions I have come to trust over time).
First up is a combination head phone/speaker amp that can also be used as a line stage preamplifier.
If you search A'Gon there is a review/post of it comparing it to their much more expensive preamp (the poster owned/used both in their system).
DeKay