Tough call. To get a little louder requires quite a lot more output power and a similar increased demand on your power supply. For example, doubling perceived volume requires increasing amp output power 10-fold. So, 100 watts is only twice as loud as 10 watts etc. If you can get the speaker above the crowd, the sound will carry better. Also, Li-ion battery will give you more portable power with less weight and bulk. I think your best solution will be some type of class D powered speaker on a stand because it's just fast & easy to set up and has efficient low power supply requirements for audio output power.
In terms of sound quality, there are basically 2 main issues, the audio source quality and the speakers.
For source, forget lossy MP3, WMA etc play from CDs or ripped WAV, FLAC etc files. It is pretty much always better to use a cable rather than WiFi Bluetooth etc. connection from source to amp or powered speaker. Even if that cable is analogue audio of appropriate length, this is still better. Unbalanced audio cable should not exceed 3m due to the possibility of RF interference. Balance audio cable can be much longer as it cancels interference. If you have the option of digital or analogue cable, the choice comes down to which device has the better DAC. Perhaps you use a mic, get something professional, not a video camera mic and if outdoors use a wind sock or pop filter.
A major issue with speakers is their frequency range. Bass response is largely determined by the speaker enclosure size. Sure, there are tricks like Bass reflex porting but you don’t get something for free, there is always a trade-off. Given that it’s robotics, I assume you are looking for that elusive bass umph. I don’t think you will get that from something you carry around. Look at what the street performers are using to get the frequency response and clarity you want. Obviously, speaker size and portability are competing objectives and this requires a trade-off only you can decide. You can however go with a single speaker enclosure to maximise the bass response and clarity. Sound quality is not cheap but utilising potential can be influenced by the way you implement things like sources and height of speaker. You may be able to pick up something from a runout sale.
While the links below have more than you require in terms of mixing and may in other ways be less than ideal, they may give you some ideas for some options:
https://www.storedj.com.au/alto-kick-15-instrument-pa-amplifier-15-400w-combo
https://mackie.com/products/freeplay
Ken
