Neal, with regular IEEE 32-bit floats, if you define +1.0000 and -1.0000 as your FS rails, you *can* represent a signal that is 700+ dB louder than that, with 8 bits in the exponent.
In 2008 I presented at an AES tutorial called “Float v. Fixed” or something like that. I showed that with an 8-bit exponent, unless you need more than 40 dB of headroom, then 32-bit fixed exceeds 32-bit float in sheer S/N ratio. Those 8 bits in the exponent is too much real estate in the word. 5 or maybe 6 bits in the exponent is all you should need for audio. Then leave all the other bits for the mantissa.