Overwriting the class on a `Html.EditorFor`

by the default with

<%: Html.EditorFor(m => m.ConfirmationHeadline) %>

the output is:

 <input type="text" value="" 
        name="ConfirmationHeadline" id="ConfirmationHeadline" 
        class="text-box single-line">

As you can see, the input appends already a class attribute. Well, this should not be a problem, just use

<%: Html.EditorFor(m => m.ConfirmationHeadline, new { @class="span-11 last"}) %>

and should work... err... nope!

this will output the exact same code!

though, works fine with Html.TextAreaFor()

How can I remove the class text-box single-line from ever appear so my own classes could be appended? any T4 template I should edited?

Thank you for all the help.