Speakers sound terrible, tinny, no bass

As the title indicates.

Car has a solid kenwood head unit, none of the settings on it have any impact. tried two different sets of speakers to no avail. The speakers are mounted pretty shoddily, which I understand will not let them perform at their best, but they just sound awful so its something besides that. What wiring f-ups by a previous owner could have caused this?

Any thoughts?

Thanks guys.

Out of phase? Sounds like maybe one or two speakers are wired backwards. Is there any gaps in the mounting surface? If there is fill the gaps in so there is a true front and back with the speaker.
Good luck.