Here's how you fix the "boomy bass" issue on your Pioneer A4. This is a method I haven't seen recommended elsewhere.
From 2012 to 2013, Pioneer manufactured a line of fantastic network speakers: the A1, A2, A3, and A4. The top-of-the-line A4 model has fantastic sound and received rave reviews everywhere, and it was The Wirecutter's pick for Best Airplay Speaker until Pioneer discontinued it. The Pioneer A4 supports Apple's AirPlay as well as HTC Connect, DLNA, Spotify Connect, and and of course your standard 1/8" headphone jack audio input as well.
There's one drawback to the A4: while it does have best-in-class sound, the bass can be overpowering, as Amazon reviewers have noted.
Most people recommend fixing the problem by adjusitng iTune's audio EQ settings, or by simply stuffing a sock in the bass ports in the A4's rear. Those methods both work but rob you of the A4's full potential.
A Better Solution
You can acoustically isolate your A4 by placing energy-absorbing rubber or foam feet between the speaker and the surface it rests upon. This gives you crystal-clear, powerful sound including bass that I wouldn't have thought possible from a unit this size.
My favorite material for this job is Sorbothane, which is specifically designed to absorb vibrations. It's the same material some people place under their washing machines. Other types of foam may work as well, but you're on your own.
- Get some Sorbothane. For the most value, you can buy it in sheets and trim it however you like. Or you can buy it in pre-cut discs or bumpers.
- Place the Sorbothane between your A4 and the surface it's resting upon.
- How much Sorbothane? Depends on the surface. I used four 1" square pieces of 1/4" thick Sorbothane.
Ideally, you should be able to place your hand on the surface next to your speaker, play a song with powerful bass, and not be able to feel any vibrations. If you do, consider adding more Sorbothane until you're happy with the sound.