At first, I was convinced that I NEEDED a tube pre to warm up my signal chain, and went through the usual suspects of entry level, starved plate tube preamps thinking I'd find something that sounded warm, fat, etc, etc, etc.
If you want warmth, go for something old that is transistor based. An old Altec may be the same price as a brand new starved plate tube preamp, and will probably come closer to that elusive "warmth" we all talk about.