Those speakers have a frequency response of 40-20kHz at 6dB down. So, it's not your set-up (Amp) that has a bass problem, it is the speaker.

The Decware blackbox product designed to restore bass response is the ZRock3, not the ZBox.

I would recommend this:
1 - ensure your amp and tubes have 200+ hours of break-in play time;
2 - give your new speakers 200+ hours of break-in play time;
3 - Then see how satisfied you are with the bass response in your listening room - it will probably sound amazing;
4 - Add your subwoofer (better yet a pair) to restore the lower few octaves that your speakers are missing and to correct any other frequency loss nodes unique to your room;
5 - Lastly, add a ZRock3 to restore the bass and heft that may be missing from your source component (ie.DAC).