You can disable the DX setting the Via and/or Warning HTTP headers respectively with:
set server factory h v disabled
set server factory h w disabled
these are global settings, you can adjust these per cluster too with:
set cluster <cluster name> factory h v disabled
set cluster <cluster name> factory h w disabled
The DX adds these as it is acting as a reverse proxy; these headers notify clients they don't talk directly to the server but through a proxy as per the HTTP RFC.
After disabling these you can use apprules to insert your own Via and/or warning header as required e.g.
PTH: url starts_with "/" then insert_reply_header "Via" "The long way"
You can use apprules to modify headers set by the target server, from your output it looks like you are using nitro.apprule or a modified version of it. You can view this default file with 'show file nitro.apprule', within are statements that hide the Server header e.g.
#PTH20 PTH: reply_header "Content-Type" contains "plain" and reply_header "Cache-Control" not_contains "no" and reply_header "Cache-Control" not_contains "private" and reply_header "Cache-Control" not_contains "max-age" and reply_header "Pragma" not_contains "no-cache" and reply_header "Expires" not_exists and http_reply_code equals "200" and query_string not_exists then insert_reply_header "Cache-Control" "max-age=600" and update_reply_header "Server" "Concealed by Juniper Networks DX" and cache "600"
You can either modify your applied apprule to set the Server header as you wish (or preserve it by deleting the relevant line).