The direct translation is 'permanent direction' and I do find this term used in web pages about symphonic music and conductors. In many descriptions of orchestras, as well as descriptions of individual conductors, I find the terms "permanent conductor" and "artistic director" together in the same page. Likewise, in Italian, there are over 2,000 google references that contain both 'direttore stabile' and 'direttore artistico". So, depending on your context, I think either 'permanent direction' or 'permanent director' or 'permanent conductor' would fit.

--------------------------------------------------
Note added at 43 mins (2008-07-09 03:29:29 GMT)
--------------------------------------------------

A few more example sentences:

"From 1976 to 1979 he was Turin Royal Theatre Artistic Director, from 1981 to 1987 he was the Artistic Director and permanent conductor of Milan “Musical ..."

" Maurizio Barboro is currently Artistic Director of the prestigious International ... He is, furthermore, the founder and permanent conductor of the ..."

"... had he not been invited to undertake the permanent direction of the Gewandhaus concerts at Leipzig, ..."

" I found a fine orchestra under the permanent direction of the distinguished Swiss composer Volkmar Andreae, Director of the Zurich University and Principal ... "



--------------------------------------------------
Note added at 46 mins (2008-07-09 03:32:20 GMT)
--------------------------------------------------

Oops ... I put in the same one twice! Here's another example:
"... until the autumn of 1929 when Bruno Walter took over the permanent direction of the concerts ..."