Hi everyone, got a little writeup for you.

Premise. As you may guess (or, if you don't, means you're OK with the sound on your laptop after the latest Win10 updates, you're on an older version, or your laptop managed to avoid this error), apparently the Windows 10 1709 update broke DTS Studio Sound on Harman Kardon-equipped Toshiba laptops (this at least affects the P50-A series). Honestly I didn't try installing it again on the very last build because my workaround hasn't failed me (yet), but I remember that DTS Studio Sound still threw out a "Failed to find APO endpoint" error whatever "IDT High Definition Audio CODEC" driver version I'd choose (there are several available AFAIK) some time ago.
Problem? Very very tinny sound, no bass at all, unpleasant to the ears even. You know your massive (quite so for a laptop, not kidding) speakers can do better, even without a subwoofer.
Anyway, not too many people have heard of Equalizer APO. Even less people have heard of various graphical interfaces meant to make it usable for an average Joe. Too bad because it's a life-saver in so many cases.

Solution.
1)
Equalizer APO is a very nice piece of audio control "intermediate" software (sorta like a permanent equalizer). It launches automatically in the background on startup. You can get it on Sourceforge. I'm using v1.2.
Of course you can configure the EQ preferences to your taste right in the APO dashboard but on its own, Equalizer APO seems to be a pretty raw program (like a backend, even an API if you will). I had some trouble with the EQ setting knobs right here (it showed an error if I remember correctly). This is what different kinds GUI were made for:
2) One such GUI is called PC Equalizer. Another good variant would be Peace Equalizer I believe. Didn't try it out. It's a graphical interface for Equalizer APO with almost the same knob system but a tiny bit different interface. What it does is that it manages the APO settings conveniently, allowing the user to create custom profiles. It's also available on Sourceforge, and v1.3.1.11 is what I'd recommend (it isn't said to have Artemis pre-bundled, unlike the newer releases).
3) After tinkering a bit with the EQ inside PC Equalizer, I made a profile which I believe would the closest possible match to the original super-boomy DTS Studio sound. It may be a bit quiet for some (I didn't enable volume normalisation) but I feel it has a very decent amount of bass.

Ladies and gentlemen,presenting to you my custom profile,suitable at least for model P50-A. Get it here: https://www.dropbox.com/s/t1ao7u7thp...reset.txt?dl=0
You'll need to copy this text profile to the preset subfolder inside the PC Equalizer directory. Normally, on a 64-bit system you can access it by typing %ProgramFiles(X86)%\PC Equalizer\data\preset inside your File Explorer window. On a 32-bit system, remove the (X86) bit and you're good to go. It is, of course, only true if you didn't modify the installation directory during setup, change some variables manually, etc. Please report if it (or something else from this post) doesn't work for you.
Alternatively, you can just open the file and alter the knobs in your copy of PC Equalizer accordingly (you'll have to switch to the 20 B mode on the "Equalizer" tab). Don't forget to save the preset.

Surely, nothing stops you from experimenting on your own. Just don't delete System32 or format your system drive in the process because these kinds of infinite "space compression" are only infinite due to their compressing the files literally to infinity, rendering your Windows installation unusable.

And of course, Linux users won't have any trouble with optimising the sound at their taste as well. PulseAudio (please "correct if wrong" ) + graphical EQs are at your service.

No affilliation.