This isn't gonna be a technical review with distortion and frequency response graphs, or anything, so keep that in mind. I'm just sharing some of my thoughts on the products based on casual use.
All of these amps are used to power my Bowers and Wilkins 607 speakers while also connected to my Klipsch R112SW. Source audio is from Spotify on my Macbook Pro connected via aux or usb.
Build quality is superb, i'd say its the best of the bunch. The chassis is sturdy and has weight, all the buttons and knobs are tight and tactile. The sound quality is fantastic in my opinion. From low to high notes, everything is there. Music is very detailed. There's no audible hiss at all even at max volume. The issue with this amp is that the subwoofer output is poorly designed, the volume is very low compared to other amps and it sends out a pulse to the subwoofer whenever you turn off the amp. The bluetooth works well. Also i really like the display on this unit.
SMSL Q5 Pro:
Feels very cheap. Volume knob feels loose and the display looks tacky. Sound quality not as good as the Topping but it's good overall. Treble and voices don't sound as vibrant and bass doesn't get that deep. Subwoofer output works well. A bit of white noise hiss when you turn up the volume past 50. Not sure if it's my unit, but when i'm connected to it via USB to play via the onboard DAC on the amp, i hear the audio clipping and making weird clicking noises, no matter what volume i'm listening to on the mac. For the price of 140, i don't think it's worth it. It's an okay unit and it does the job.
Feels and looks very similar to the Q5 Pro. Buttons and knobs feel cheap and the display looks tacky. Sound quality is great and almost on par with the Topping. Highs are crisp and bass is deep. Little to no white noise. Provides the most power out of all these amps. It drives my speakers well and it gives a ton of volume. Subwoofer out works as intended. Only real issue i have with this amp is the design of it and the volume knob is kinda finicky. Very good amp for the price.
Very high quality build. The chassis is nice and stiff and has weight to it. It seems very well put together. The display is also nice and crisp, you even get to change the color. Sound quality is extremely CLEAN and i might say it sounds the best among these amps. Great frequency response. Bass sounds the deepest and the highs sound crispy. I hear a ton of detail in my music. USB, bluetooth, aux all work perfectly. Absolutely no complaints about its functions. The only thing i dislike about the amp is the odd form factor. Overall it's a great amp and well worth the price.
Final thoughts: If there was only one that I'd keep, it'd be the SMSL SA300. It does everything you need and provides great sound. I really wish the Topping MX3 worked better because it looks great and sounds great, but the subwoofer output issues made me dislike it.