I was kind of reluctant getting these speakers but after a weeks use I'm absolutely glad that I did. For such a small package it's so much louder then I expected, volume goes up to 40 I find 10-20 is loud enough for me but great to know I can get more from them. Can adjust the treble and the bass to suit you and your music, bass is really powerful but not overpowering, again how on earth have kicker's got so much from such a small package. The quality of the sound is amazing, I'm no expect but these sound good. A remote is provided, it provides basic controls, I mostly use it to change volume and skip tracks. Very inconvenient to use the remote to navigate your music library on your iPod/iPhone. It's aggressive looking, not attractive as other speakers but have a quality build about it. They are not portable but are light enough to carry around comfortably.
I brought it for my iPhone 4 and it works perfectly fine with it, I haven't needed to put my phone into airplane mode, a very nice surprise. Another nice surprise is the dock chargers my phone even when listening to music. I honestly can't think of any issues I've found (yet), when receiving a call or text it just stops the music, exactly what you would expect it to do. You got an aux connection so most mp3 players can connect to it, there's another connection but I don't know what that is.
I don't know the difference between these speakers (ik501) and the other ones (ik500) but I got these ones because its the newer model. I think there is only one other speaker dock that matches this one in terms of size, sound and quality is the b&w zeppelin which looks awesome but cost £400. These speakers are a great, I would totally recommend them to anyone, a huge range of events these could entertain from garden parties to house parties or like me just have them in your bedroom.