Just echoing what others have said. This isn't about tone, it's about the audio quality of your videos. You have to make a choice - stick with the iPhone for both audio and video, or do something that's more complicated.
As you mentioned the simplest step forward is using a different mic or recording interface on your iphone, such as the Blue Mikey. This WILL help. I have one. It's much better than the built-in microphone. It won't completely solve the problem, though, for the following reasons:
1. The mic will still be with the camera, and, in turn, far enough away from you that it will be picking up all kinds of room noise, etc.
2. The basement you're in with the concrete walls probably house sound bouncing everywhere, which just exacerbates #1.
I actually believe that the best recording you're going to get from the iPhone (other than actually recording in a properly treated room) would be to do it outside, and just go purely acoustic - no amplification.
Regardless of where you're recording, though, assuming you really want to stick with the iPhone and not mess with any syncing of audio and video after the fact, might I suggest the following:
1. Go ahead and get the Blue Mikey. It's cheap enough at $50 from Amazon. I'd get the newer version (2.0) as it's not much more expensive.
2. Get a doc extender cable for your iPhone. This will allow you to connect the Mikey to it, yet point the Mikey at your sound source, while pointing the camera wherever you want.
3. Get
ONE OF THESE. You basically put this little self-adhesive metal ring around the camera on your phone, and you can easily attach/remove this magnetic wide angle lens. This will allow you to put the camera closer to you, while still getting the same field of view. Closer to the sound source = less room noise.
You're never going to get pro quality from an iPhone, but I do think you could improve things quite a bit by doing these at a fairly reasonable cost, and not much more hassle than you're dealing with now.