Troubleshooting this stuff can drive ye to drink! Assuming your source data sounds A-OK with a hardwired connection, then you're left with either a flawed BT implementation/codec on your playback source device, or flawed BT implementation on the 430, or even a bad (soldered?) connection between the 430 and speakers? Since you now have new speakers, maybe we can rule them out of the equation. I feel your pain. . . I hate it when this stuff throws up.
I'd try this if possible: (start with a refreshing beverage of your choice - mine might be alcoholic) If you have a laptop that is Bluetooth capable, see if you can pair that into the 430 (I have no idea if this is even possible with the 430, but BT is BT at base level so it should work). Play the same source data (music) file from the laptop and note if it sounds any better or same junk you've been getting. If audio is better, then you know the problem is most likely in your iPhone 4G.
A known problem involves trying to use Bluetooth while WiFi is activated on the iPhone. They both share the 2.4HGz frequency. Way to go, Apple! FWIW, iPhones have been notoriously flaky with all manner of Bluetooth headsets, evidently a less than state-of-the-art implementation of A2DP protocol.
With regard to other Bluetooth devices (headphones, etc. , not necessarily a 430 or 730r), some have reported getting better results if re-pairing to the receiving device, making a new passcode.
Some have reported low-bit-rate audio rips have compromised audio similar to what you have described, whereas higher bit-rates have sounded significantly better.
At about this time, I am ready for a second beverage of choice, and would summarily blame. . . ??? Apple? Chrysler? Canada? Like I said, I hate troubleshooting this stuff. I'm picking up a guitar, plugging into a plexi Marshall, and crankin' her up to "eleven". No Bluetooth, no worries!
Good luck, and keep us posted. If I had my Summit (by the looks of things, next Winter, a year after ordering) I'd probably be suffering alongside ya, and might have better advice!