I think the headline needs to fit recipient, and the best headline has a connection to the reader.
So if I was sending a letter to carpet cleaners to sell marketing letter templates I would use the headline....
"Heritage cleaners made an extra £1500 last week...... Do you want to know how?"
Then tell them how my sales letters worked so well they got an extra £1500 in business from just one mail-shot.
People relate better to what they know so will respond better to this approach, so send a letter to hotels with a headline like this ....
"Only 3 hotels in Swindon know this odour removal secret... Are you one of them?"
Everyone what's to know a secret
do you think a hotel owner would read that headline and not continue reading?