The C2200 is a nice pre. You shouldn't have a bass issue with one if it set up right.
Tubes can make a huge difference, especially if the OEs are tired. Don't think Mac used great tubes -- they used good current production tubes that were then available. Back then, most new production was mediocre sounding. At least the hard-sounding Sovteks of that era didn't explode like the old Sino bottles did. You can do better.
Those are a cakewalk to roll bottles in. 12AX7 and 12AT7. Only 4 in the line stages. And choices are plentiful. If you want to improve the bottom end, I would not run Teles in it, and would definitely never use new production of any kind (there is no need to in an X7 or T7). There is a mountain of vintage still out there. Save the diamond bottoms for the phono stage, where they tend to really excel. The ribbed plates will give you a little more bass than a smooth plate, but neither of them are going to compensate for a bass deficiency.
Try something like an RCA or Raytheon black plate. Those are great all-rounders, with an emphasis on rich, strong bass and mid bass. If for some reason those come across as a little too constrained on top, then perhaps a Sylvania black plate (those are early 1950s production and much harder to find, however). A later long grey plate RCA will give you even better bass than those three, but possibly at the expense of too much elsewhere. No later Sylvanias/Philips or later GEs (but an early Ken-rad will do nearly as well as those other blackplates). A Holland Amperex is not going to give you enough extra bass, but will give you everything else. Very generally speaking, try to stick with an earlier long plate if you can, it will give you a better chance at what you need.
The holy grail when I need more bass and presence with no loss of sparkle on top is often a 57 to 59 Blackburn Mullard. Warm and rich. Generally powerful sound. But those are becoming hens' teeth, cost a small fortune when you can find them, and you are not getting any of mine. The later ones after 1960 are nice, but do not have the wide open presence and low end authority of the early ones. A cheaper alternative is a Hytron/CBS, a real sleeper which comes close to that Mullard long plate at times in the right circuit. But they can be hard to find, too.
Good luck and happy rolling.
