OK, if there was an 'explosion' and smoke coming out of the RCAs as you describe, there will be evidence. I see no evidence. There is a faint possibility there was a short below the barrel connector to the chassis or pin to PCB- check that. Smoke generally means vaporised solder/copper and that usually leaves a 'film' on the PCB/component or adjacent casework.
Perhaps the power adaptor connector itself shorted in the lead/plug and the 'explosion' and 'smoke' was actually outside the unit.
Based on your excellent photos, I wouldn't be overly concerned about using the unit, power supply connector check notwithstanding.
First of all, thanks for your valuable observations and shedding some light on the subject based on my images, I appreciate it a lot.
About what you say regarding to the explosion and the smoke, it was indeed visible (and strongly audible like a bang) on the outside of the case as a small sparkling beam of light just near the RCA inputs, followed by odorless smoke in that zone that vanished in a matter of seconds.
I had the courage to use it shortly after the incident as I described in the first post (with a cheap Koss KPH30i that I have lying around) and certainly everything worked without any problem despite the scare I got.
Anyway, can this show some flaw in the design or can it have a possible explanation, in order to avoid it in the future (or to take certain usage considerations)? I've only had a Schiit Magni 3 a few years ago (connected directly to the RCA outputs on the motherboard of the PC I had at the time) and a DX3 Pro after that connected by USB, neither gave me any similar issues or scares.
The only things that have changed with this Topping L30 were:
-It was very close to the case of my new PC rig, right next to the PC's PSU (however, that area does not generate any heat, on the contrary, I have a radiator that expels air towards the front).
This close.
-I had the RCA cable connected to the L30, but the 3.5mm termination (it's an RCA to 3.5 cable) was disconnected and lying on the desk mat, since I only use it to connect the Qudelix-5K as a "desktop" DAC/DSP.
Like this.
The only thing I want is to avoid that something like this happens again, be it with this amplifier or with another one that I end up buying, since it was audible throughout the house and scared the hell out of us, especially me who was nearby.