I doubt sensitivity is the issue and the load might play a bigger role. 40 watts will get you around 103 db at 1 meter and that isn't anything to scoff at. If the amp can't handle the load of the speaker though it won't be a pleasant experience. If you are looking for reference levels then that's a different story and you'd need a high powered amp with high sensitivity speakers.
Does the amp have different taps? If so you might try playing around with them.
Does the amp have different taps? If so you might try playing around with them.