It could simply be due to the ceiling speakers and/or how they’ve been installed. Which model are they? Have you checked that they’re been wired correctly, in phase?

You do of course have the option of adding a Sub to the Amp if you wish. 

Thank you for that. Yes, the vendor mentioned about getting a Sub, but to be honest, I didn't want to spend  extra money on it. I'm unsure what you mean by " in phase ". The vendor also mentioned that the ceiling acted as the bass. Is that accurate? 

Think of the two drivers stepping forward together, then stepping back together. If instead one goes forward while the other goes back they will tend to cancel out, especially at low frequencies.

As an experiment, swap over the two speaker wires RedBlack at the Amp for just the Left speaker. See if the bass improves.

In phase means “both speaker cones move the same direction for the same signal. You can just flip one pair of speaker wires and see if it sounds better or worse. Mark all four wires once you have the best sound.

The ceiling isn’t acting as bass. What you may have is open back speakers and the ceiling is acting as the speaker cabinet.

Do you have more than the two speakers connected to the Amp?

If not a Sonos Sub maybe another brand, even used to get the price down without hurting the sound too much.

In the regular speakers that are familiar to you the cabinet is carefully designed to compliment the raw driver. In the ceiling mounted speakers, the “cabinet” is haphazard and often diminishes the bass. Also, in a typical ceiling the two speakers can easily interfere with each other in the bass region. Many times you can improve the bass performance by stuffing the ceiling with mineral wool or fiberglass.

With respect to “phase”, think of each speaker as being a diaphragm that creates a pressure wave on one side and an anti pressure wave on the other. The cabinet’s job is to keep these two waves from encountering each other and cancelling the bass. In most music the bass frequencies are essentially mono. “Phase” is a reference to which way the speaker cone (diaphragm) will jump when a (+) voltage is applied. If the left and right speakers are out of phase, each speaker will tend to cancel the other speaker’s bass wave.

Many people are sensitive to out of phase situations simply by listening, and many are not. It takes a little experience to learn how to listen. Ratty and I have enough experience that we can step into a room and almost instantly notice a phase problem, but we have some experience under our belts. Another way to approach identifying phase is to use a flashlight cell. It is an industry standard that at the instant a {+} terminal of a flashlight cell is connected to the {+} terminal of a speaker the speaker cone will jump away from the “basket” (frame). If you can view the speaker cones of your ceiling speakers you con use this technique to check the phase of the speakers. It is very important that the two speakers are “in phase” (both jumping out at the same battery polarity).