Best MC step up transformer

Transformer Design is based on perfection and knowledge, based on that , their differences are huge. Some of the better ones are made from Cotter, Fidelity Research and Jensen.
Most out there do change the sound (color), it can be depressing to work with them, I had also the ones from Koetsu, EAR ... really everything but neutral. Good luck. The Kondo SFz is indeed one of the better ones, but the pricing is "special" :-)
You can try to find a technician who replaces your Jensen with those from Jensen's best (no Manufacturer offers top quality here, why should they cut their profit...)..or you buy a better cartridge :-)

I have the Bob's Devices flagship SUT... It's $1200 and results have been dramatic! See my review posted here (link below).

If you're on a budget then the Hagerman Piccolo is hard to bear, but it's not a transformer. It's a head amp using op-amps to boost the signal.

Have you seen Dave Slagle's transformers? Very nice. Custom wound to match your cartridge.

I use a Raphaelite PM 1.1. I bought it through Pacific valve, and although they no longer carry it they might be able to get you one. About $600, and phenomenal in performance. It will give you a deeper bass response than most solid state MC stages.

Any SUT delivered without a Data Sheet about it specs is wasted money (Most have no sheet or they are wrong), I would give any SUT to an external Lab for an independent check ... I saw so many where the Tech scratched his head and asked me "Paid more than $50,--??