A transformer is a transformer, and any of the types will do the job.

The basic difference between an autotransformer and a conventional one is that the autotransformer does not provide that important "isolation" thing between the primary and secondary, as both are part of the same common winding. But as a result of this they also gain in efficiency & cost as the iron-copper requrement & losses reduce by half. Capacitive losses and resonant points also diminish quite a lot.

The OP already has a conventional output trafo built-in to the amp and terminated at 200 ohms. So ideally speaking anymore isolation is unnecessary, and a suitable autotransformer should suffice the needs (if one finds one). But the UTC LS-33 that "AB" has sited is of very good quality also, and will do the job equally well if not more. May be it is a bit costlier compared to a similar but of the autotransformer variety.