The ones at Parts Express are cheap Chinese knock-offs of some of the other Televes designs. They are not Televes antennas.
Peak gain on the 91XG is up in what is now the cellular bands although channel 51 gain is around 17.4 dBi(
http://www.antennasdirect.com/cmss_f...s/91XG-TDS.pdf).
The DB8e also has a peak gain of around 17.4 dBi on channel 51 but offers much improved gain at the lower end of the UHF band. (
https://www.antennasdirect.com/cmss_...s/DB8E-TDS.pdf
)
The Televes Boss Tech series uses an automatic gain control feature in the amp to vary gain depending on the strength of the signals at its input. It's been around three or four years, but they've been tough to find in North America (Televes is located in Spain) since they hadn't focused on the US market until recently. I've yet to see one.