Just to correct some inaccurate information:
The Flex 6500 and 6700 overload at +9 dBM with preamps off according to Flex (Rob Sherwood measured it at +11 dBM IIRC). The Flex 6300 overloads at +7 dBm with preamp off.
73, Bob, N7ZO
Yes, this is correct. Further, as Rob states, the Flex does need the preamp to hit its noise level of roughly -130 dBm. Without the preamp, the Flex's noise floor is roughly -110 dBm. In other words, you need the Flex's preamp.