Step up device or new cartridge?

Huh, I came real close to swapping my Lehmann Black Cube for a BAT phono stage to use with my VK30. I guess I should be glad I didn't go through with the swap. You could sell your BAT phono card and get a higher gain outboard phono stage like the Cube, or...pick up the Benz or perhaps a high-end Grado. The beauty of keeping your current phono stage is it is an add-on to the BAT preamp, reducing clutter and interconnects by having the RIAA/step up internal to the preamp. I'd lean towards updating your cartridge and enjoy the music.

I assume you have switched the card to the hi-gain setting. If so, it depends on how much you like the cart. I do believe that it is a pretty low output mc and would benefit from a step-up device. The advantage of buying one now would be the ability to use any cart in the future. The advantage of going with a new cart is that the Monster is probably pretty old and even with low hours may be nearing the end of its useful life vis-a-vis the suspension.

You're in the situation where you have two counterfactors:
High output compromise in transparency and detail to the low output cartridge and the secondly, step-up transformer brings harmonic distortions to the signal.
A good step-up transformer will cost more than Benz Glider for sure. In most cases you should use the high output for built-in phonos.

Another thing to keep in mind is that step-ups can often cause grounding problems and introduce hum into your system, and that's sometimes a devil to get rid of (I've had this problem in the past). Plus, unless you spend a good deal of money for an Expressive Technologies or similar unit, many step-up transformers can tend to add their own distortions and degradations to the signal. And then there's the cost and tangle of additional interconnects, the possibility of impedance mismatches... I think your best bet would be to update the cartridge (I used to have that cartridge, and its good but I think it can be matched by some of the modern higher output designs) unless you can get a different phono card for the BAT with more gain (don't know if this is available).