The KT88 tube is probably your best output tube for bass; the KT88 also has a nice bit of punch to it on both frequency extremes. Others, such as the EL34, will tend to soften the bass.

However, the KT88 also does not have quite as nice of a midrange as does the EL34 - so you may prefer the EL34 for the rest of the sound.

I suggest calling Kevin Deal of Upscale Audio. He has always been of great help to me when choosing tubes.