amp upgrade

using a two channel amp/pair of fl/fr speakers w/o center speaker, you will need to go into each movie disc and find the audio settings and change it to 2 channel stereo. 

they default to some surround version, usually dolby 5.1. In those default cases, all the dedicated center channel information will be missing (totally). Much voice info will simply not be present unless you tell the bluray player to send the center info to fl/fr. 

after fixing above so you are receiving center info in the fl and frL

for very low volume, you need to boost the bass just a bit, our ears do not hear bass well at low volumes

https://en.wikipedia.org/wiki/Equal-loudness_contour

some equipment has night modes, that compresses any peaks so normal dynamics that would produce momentary volume changes are 'dulled', not a good solution if you can find a better way, but, better than nothing.

The amp forsure can help but imo theQuality of your Starting point is maybe more important  for good or bad your digital signal starts their and if not good to start with cannot be bettered upstream . Buy a good quality dac .

On a budget I highly recommend the Denafrips Aries 12th anniversary dac,

then for onk6 $540 the very good matching Iris DDC oversampler which not only clears incoming data bet with the I2S connection is the. Best type of connection

but even more so the temp control clock in theIris is better then even the latest 12th  pontus.same clock as in the Venus , and through our audioclub did what other reviewers don’t do put decent power cords ,and upgrade fuses,on the Iris to save cost theHigi tuning Supreme Copper fuse, and the synergistic purple on the dac 

fori2S cable we used a Wireworld star light ,but even their $60 hdmi thepin out matches ,other brands may not. And after roughly 200 hours ,sounds very good in some areas better then the latest 12th pontus more fleshed out detail and still very natural , the latest Aries 12 is just over $1k , and then you can always add the Iris DDC later which is a big plus ,my wife’s loves it in her system.

thanks, sorry I was not clear. the movie source is not blue ray, they are mostly streaming 4K from the smart TV which is connected to the amp via optical. The TV converts from DTS5.1 to 2CH PCM stereo. Maybe the issue is the TV DSP and downsampling to 2CH. I can rule this out by trying a blue ray movie and setting the blue ray player to 2CH PCM and see if it makes a difference. 

the equal loudness reference is very helpful, it seems the low frequencies need quite a bit of boost as the amplitude drops. I am going to try this, I have heard some systems that perform much better at lower volume but maybe they had more bass to start with. 

Unfortunately part of your issue is the speakers.  Some don’t really wake up until higher volume. My Dynaudio Contours (original series) are like that.  They love the juice. I recently got the Heritage Specials and have been shocked at how great they sound at lower volume.  Same amp, different speaker. 

Coincidentally, What Hi-Fi also made a comparison between CXA80 with Arcam (but different model A19), pinpointing these shortcomings you have mentioned.  I am not sure if you  have seen this article before and influenced by it though ...

https://www.whathifi.com/cambridge-audio/cxa80/review

Lots of good advice here.another would be to switch out the optical cable for a digital coax.  Optical is good for video, not so good for audio purity.

Buy a receiver and a center channel speaker. Receivers are cheap now. Use the theater bypass on your cxa80 when watching movies. No way around this with a 2ch amp. 

I looked at AVR I have ceiling speakers that can function as 4 channel, the issue is that I want the integrated amp to drive the tower speakers (most cheap AVR sound awful, I had a pioneer "elite"  and ditched ditched it a while back). furthermore cheap AVR's don't have typically pre out except for sub. What I need is a DTS/surround processor that would take the TV HDMI-ARC and process it into left/right/center/surround and output it via pre out so I can connect it to the stereo amp for the towers and the mono amps for the ceilings speakers. to my knowledge only one such equipment exist made by emotiva audio, but there aren't many reviews of its quality  https://emotiva.com/products/basx-mc1-13-2-channel-dolby-atmos-dts-x-cinema-processor?utm_campaign=gs-2019-11-07&utm_source=google&utm_medium=smart_campaign&gclid=Cj0KCQjwmN2iBhCrARIsAG_G2i6Id_t07wXE5UZX1ke8nWuITpDho_mcZLF9caSiV9PG4eNiReZCQ6saAoiPEALw_wcB

Thanks for the review article it does echo my experience. I recently got an Arcam sa20 amp and st60 streamer with DAC and sound quality improver noticeably. Including low volumes. The other amp was just not a good match for my speakers. 

Although B&W D4 is rated 89 dB sensitivity and 8 ohms (seemingly a relatively high sensitivity speakers), larger wattage with clean high current amp is required to reach its potential. In particular, if you want to explore its the low ends (down to 24 hz) and extends its renowned soundstage, you probably need to upgrade the amp.

But, before doing so, try the followings first (ignore these if you have already done so). I have Cambridge Azur 851a which has similar features with CXA80. First, always use "Direct" mode to bypass the tone control, which is supposed to render better sound quality. CXA 81 actually gets rid of tone control. Also, connect the sound source (like DAC; CXA80's internal DAC is not that impressive) to the Integrated using XLR. I attain better sound quality (more dynamics etc.) too not just louder.