Hey OM:

Yes you can use only 12AX7's, you might need to cascode them to reduce
the miller effect so the disadvantage would be you are going to use
twice as many tubes.
There's lots of audio circuits out there, but for RF you would have to
use a 12AT7 the input capacitance of normal 12AX7's sux big time
unless you go thru a bunch of them and find sum hot 12AX7's
In the old days we used 12AT7's in garage door opener receivers they
used the low power band in the low end of VHF before it went away.
They were made here in Detroit by VEMCO.

HF band should be no problem, see this link on miller effect of
12AX7's:

http://www.aikenamps.com/MillerCapacitance.html


Still, my question remains: can one build a decent superhet radio out of
nothing but 12ax7 dual triodes?

--
David Griffith
* --- Put my last name where it belongs