I have the same setup, Focal Kanta 3 with Evo 400 preamp but with Evo300 monoblocks.
First off, you are going to get some replies that Primaluna has plenty of bass. I agree with this. However, I don’t disagree with you that the Primaluna could have ‘more bass’. But I believe that this is NOT in quantity, but speed. Sustained bass notes (like from that of organ pedals) will have plenty of pitch definition and heft. But the initial attack, or transient speed, may be lacking compared to some other amps. For instance, I also have an NAD M23 Class D amplifier here, which when paired to the Evo400 preamp has faster bass, and when paired to another fully solid state preamp has even faster bass. So I believe part of the ‘problem’ is the preamp as well. For most music, the primalunas are completely sufficient for my tastes in the bass department. For some other music, like with drum kit and faster transients, I prefer some solid state in the mix. And solid state amp and preamp is faster still. I don’t think subwoofers will change much of any of this. Tube rolling won’t change much either. Actually, for bass speed I think the EL34s are as good or better than any of the other tubes I have tried, which includes KT-66,77. 6L6GC are even slower.
For further comparison, I also happen to have a Denon integrated amp here that has -less- speed than the Primalunas as far as bass goes, so some solid state amps may be slower than the Primalunas. Lastly, I once had the previous version (dialogue) HP series (I.e., 8 power tubes per amp like the evo400) with KT120s driving Magico A3 speakers. This combination was s-l-o-w in the bass speed. So I believe speaker choice has a lot to do with it as well. The Kanta’s work very well with the Primalunas. When I first bought my dialogue series, I had heard them at a dealer paired with Vandersteen Treo speakers, and the bass was excellent. In fact, I brought in a class D integrated amplifier to compare (Jeff Rowland) which was much more powerful, but was boring in comparison with these speakers and had no more bass. So again, I think speaker pairing and synergy is important. As I said, from my experience the Kantas are a great match with Primaluna. If you find them lacking, you might try playing more with speaker positioning. If that doesn’t work, try some solid state gear to hear if there is an improvement.