There are tube amps that provide a preout, including the Cary Sli-80, but that one may be a little above the price range used. I'm sure there are others.
An alternative is to get a sub with speaker level inputs. There are lots of those, including some relatively inexpensive ones, and that's the recommended connection for some high-end sub manufacturers like REL and HSU. I have a small Mirage S8 that costs around $350 new and works very well with its speaker-level inputs.
An alternative is to get a sub with speaker level inputs. There are lots of those, including some relatively inexpensive ones, and that's the recommended connection for some high-end sub manufacturers like REL and HSU. I have a small Mirage S8 that costs around $350 new and works very well with its speaker-level inputs.