The problem with delivering something that appears to be a bit of test
equipment, is you are competing with used test equipment. When it comes
to test equipment, 20 year old HP wins over brand spanking new brand X
every time. Most of the used gear I've bought over the years is still
used in real labs. I could get it calibrated if I had the need. I have
to think really hard if I ever bought any new test equipment.

If your box was digital through and through, then maybe you might get
some sales. I'm not sure which Exar chip you are using (BTW, I used to
work there), but the Twin-T is an analog topology and thus has all the
bad features of analog as well as the good. It should be possible to
make a completely digital AM or sideband modulator with a DSP chip and
high speed DAC. The coordic is how the sine wave is computed, though at
the time nobody really talked about it since there were still patents
on coordic processors from the scientific calculator manufacturers. One
of the best books I've seen on the subject was a masters or PHD thesis
by a Stanford grad whose full name escapes me. First name Ahmend, which
might as well be Joe or Bill nowadays.

A coordic is a lookup table technique that converges on the the sin and
cos. You make a sine wave generator by accumulating phase, then taking
the sin/cos of this phase value. If the phase accumulator were 8 bits,
you would use a scale where 256 is one revolution. This way as you
accumulate phase, the sine function automatically wraps around after
reaching one revolution. You control the amplitude of the sine wave by
the initial value of the coordic. It's really much easier than it
sounds, at least the algorithm, Programming the DSP is another story.

I suspect the Wellbrook has a jfet amp because of the positive ground.
I think today most CATV circuits are bipolar to simplify the biasing.
For certain frequency ranges JFET has lower noise.You can parallel them
up for lower noise. There are a few companies still serious about
Jfets, but they are becoming less and less since the money is in high
bandwidth.
http://www.calogic.com/ comes to mind as a jfet company

Another idea would be to make VHF/UHF amplifiers using SiGe.