Subwoofer recommendation

I have a listening room approximately 25'x25'. Until renovations had to be made (due to smoke from a fire) I was very happy with my sound.  Now, with the room emptier I'm feeling a lack of bass.  Can anyone suggest a good subwoofer that will add bass but not alter the basic sound of the system? I have wonderful modified vintage Dalquist 20 speakers
  Looking in the 1K-2K range.