Thanks for writing in!
To change the tag archive sub title, copy _landmark-header.php file from framework/legacy/cranium/headers/views/integrity and put it in /themes/x-child/framework/views/integrity/ inside child theme. On line number 80 replace the text "Below you'll find a list of all items that have been tagged as" with your own.
By default it's not a feature offered by X Theme. However it's possible with custom development, but this would be outside the scope of support we can offer. If you are not comfortable with codes you may wish to consult a developer to assist you with this.
If you are not familiar with setting up child theme, please refer our knowledgebase:
You can download X Child Theme from following resource: