What speakers are you using? If your using lower sensitivity/efficiency speakers, you may have a problem with more demanding source material. Also, powered subwoofers work better for bass than most conventional passive speaker woofers(ported or otherwise). Still, a lot depends on the speaker design!
I would suspect your tube amps would sound really powerfull on some very sensitive horn speakers with big bass drivers. Or driving some speakers with powered woofers, or potentially some small monitors crossed over to a subwoofer!
If you're driving difficult full range speakers, of lower sensitivity, and possibly impedence, you may have problems getting dynamics out of em.
