The base model X300 had the TT K46, which is a light-duty trans intended for a dedicated lawn-mower, (i.e. grass only on a residential lot). The higher-end X300 series, like the X320 and better as the poster above mentioned, stepped up to the K58 which is a definite upgrade.
The X500 series steps yet again to the K72B, (a very heavy-duty trans), and stays there for the entire X500 line, w/ the exception of the K72A on the 540 series, which is the same trans w/ a secondary filter. All good stuff regardless.