I had a number of MC step-up. All of them sound completely different. Day and night difference.
I had:
1. Altec 15095
2. Altec 4722
3. Haufe T890 (the modern production, not the vintage one)
4. Jensen 44
5. Tamura TKS-83TWIN
6. Tamura MC20 (Sun Audio SAT-1000)
The worst are Jensen and Haufe. Really bad.
The best are Tamura.
Altecs are in the middle. Not bad.
If the load is too small (too high impedance) and sound is too bright you can solder the resistor (from 100K down to 15K) to the secondary of the step-up transformer. Smaller resistor - darker sound.