I'll second the suggestion of Black Ice Audio. They're a venerable company (used to be Jolida), they're based in the US and they have very good support. Their integrated amps have subwoofer outputs. I haven't heard their Fusion line but a friend has the CD/DAC/Streamer and loves it, and he's a very picky tube guy.
But it's also true that most well-made tube amps will not have an issue with running subs off the speakers taps.