Our sales team uses Outplay to experiment on subject lines with one particular experiment getting 68% open rate & a 12% reply rate.
This sequence was a result of running many experiments on subject lines where we used different language and different personalization techniques to find out how our prospects react.
We also experiment on the day we send these emails and the time we send these emails.
If you are looking for a tool that can help you do this, & natively sync with your Pipedrive account -
Also, remember subject lines play a part in the overall process of email outreach, you need to follow up intelligently and consistently to get better results.