I've played with a number of SA-7000s as they are a common antenna used for Emergency Alert System receivers that need to hear AM broadcast, FM broadcast and NOAA weather frequencies from the same antenna.
I agrree the SA-7000 is better than a Discone in the HF range but the Discone is a better overall antenna for 100MHz and above. The SA-7000 is a very mediocre HF antenna using a 6ft whip and a high ratio transformer to try and match the very high impedance whip to 50 ohm coax.
There is a diplexer built into the SA-7000 that combines the HF whip and VHF/UHF whip. The VHF/UHF whip is from one of their other mobile antennas and bent to get the whip away from the HF whip. There is no ground plane, so the VHF/UHF side is not as good as a similar antenna above a ground plane.
I was a bit disappointed with the SA-7000 in the last install of an EAS system and ended up taking off the VHF/UHF whip and making a dual dipole for 100MHz FM broadcast and 162,5MHz NOAA weather where one side of the dipole was on the old whip base and the other dipole elements were attached to the box and sticking downward. This greatly improved FM and NOAA reception feeding the long coax lines we were forced to use.
I've compared the SA-7000 on HF to various full size amateur HF antennas like a G5RV and 80m offset center fed dipole and its night and day difference. The SA-7000 only hears a fraction of what is really out there, especially with lousy band conditions these days.
Bottom line is the SA-7000 is a very compromised antenna for all bands and an active HF whip along with a Discone will probably give you noticeably better performance. Or better yet a PAR end fed or similar will be much better on HF if you have the space.
I bought an SA-7000 and dissected it to see what's inside and to compare it with an antenna I'm designing specifically for EAS use. If you are really sold on an SA-7000 I might have one for sale soon.
prcguy