The base of all videx clones are two hardware decisions:
- 6845 CRT controller
- 2kB SRAM as screen buffer, start address $CC00 
- number of alternate character sets
- local relais to switch from 40 column to 80 and back
The real difference makes the firmware. Try to get the STB-80 firmware,
burn an EPROM and try this firmware with your videx clone. There is a
real chance of success, IMHO.
Post by Jorge
Apple's own 80 columns, as in the IIe and IIc, that's the worst of all !
There are advantages and disadvantanges:
+ Just a single connector for the monitor, no flicker when switching
between different video modes
+ The programmer always knows the dependency between address in memory
and screen location
- Scrolling is very slow
- The other functions are also slow especially with the first firmware
of the IIe (that's what I used until getting the enhanced ROMs)
- Waste of main memory
The last point inspired me to use a videx clone in the IIe running my
own UCSD BIOS: I got more usable memory for the pascal programs.
 I'm not sure if every videx clone uses this address, but I think
there is no other choice.