The Flex 3000, if you use different software, will, as far as I know anyway, modulate with audio frequencies above 3kHz.
Yes, the bottleneck Flex used to limit the audio high end frequency was removed by Darrin Ke9NS with his version of PowerSDR. He has done many things to improve the AM experience. One example is a setup button to flip the audio phase.
When you are in a QSO with a Flex person, the guy on AM with the Flex is often peaking the voice peaks such that wavelets are generated (>100% neg. mod) and on a diode detector receiver that sounds awful. I have been in QSO, Flex to Flex with synchronous AM selected (SAM), and dropped the AM carrier and advanced the audio for more audio "punch". This is essentially DSB with reduced carrier....works very well and sounds great.
A few months ago I was on the air in QSO with Don K4KYV. Don uses a Sherwood Synchronous detector hooked to his 75A4 receiver. I was on my Central Electronics 20A station running 150 watts carrier AM. I tried DSB with only 15 watts carrier, and then I boosted the audio to maximize the PEP. Don said it sounded the same except the audio was much louder.
The DSB with reduced carrier topic is worth further discussion. Maintaining compatibility to BA receivers using a diode detector is limiting the communication effectiveness of AM.
Jim
Wd5JKO