The link above at Love My Echo is interesting, although academic. The default audio output of the Echo is "flat" frequency output (i.e. balanced sound) and if an Echo is out away from all walls and placed more in the open in a room (as it should be) there would be no reinforcement of bass frequencies unless placed in a corner, because corner placement does favor bass frequencies from speakers placed there.
I say it's academic, because the Echo speakers (and audio amp) are just too small to be capable of "real bass" output. What people often perceive as bass, when it comes to small speakers/amps like ones in the Echo, is the range of high-bass through low-midrange frequencies. If one wants to hear anything remotely closer to actual bass from an Alexa device, it is from the audio out of a Dot into a quality amp playing through quality speakers, and even the bass heard that way likely starts from mid-bass, upward. Hearing very low bass via the Alexa devices is impossible because Amazon is not sending those low frequencies to be sure, due to their restricted bandwidth. They are sending an "acceptable signal" without the lowest lows (as found on CDs,) and without the highest highs (as on a CD). Amazon is sending basically an FM quality signal at best. Anyone who thinks the devices are capable of super high fidelity is mistaken. The audio sent is quality enough for a "very good table radio" (which sounds better if run through the Dot audio out). If one wants to hear better fidelity, turn off the Amazon device and play a CD through a quality stereo amp and quality speakers. When it comes to audio out, the Echo devices are all about convenience and not about sound quality above FM radio heard on a good stereo.
My Dot playing through my stereo sounds better than the Echo; mostly because "size matters". Even so, the Echo plays music more often than the Dot/stereo does, because most of the time we opt for convenience over higher audio quality when it comes to listening to the background music from an Alexa device.