xbithack and migration from Apache 1.3 to 2

I've got a number of cases where HTML pages are enabled for SSI
processing by use of XBitHack full. In the past, these pages had
their "charset" defined in .htaccess by one or other of:

AddType "text/html; charset=whatever" html

or:

AddCharset whatever html

Either of these worked fine in Apache 1.3.*

On migrating to Apache 2, however, it appears that the presence of
either of these directives in .htaccess is preventing the SSI
processing from occurring. As soon as I disable the relevant AddType
or AddCharset directive, the SSI processing comes to life - but then
the "charset" reverts to the server's global default, rather than the
one that I wanted to use.

Guidance, please, on a good way to get both XBitHack and my choice of
charset working at the same time in Apache 2.

thanks