🚀 go-pugleaf

>   Yup. RQDX1/2/3 (M8639-YA/M8639-YB/M7555, respectively). However,
> there's a catch: These boards have 50-pin connectors which go to a
> distribution panel buried in the BA23 box. You'll have to make a
> pretty contorted cable based on minimal information in order to use
> the RQDXn.

There is a distribution panel that comes in a BA123 box that you 
can connect up to the RQDX fairly easily.  I've picked these
up for about $50 used.  You do have either plug it into a slot, 
or run power to it (all it uses from the qbus slot is power).

>   Also, RX50's are only 400K and are not formattable. I'd suggest 
> getting a 3rd-party floppy-only controller that emulates a DU and
> provides RX50 and RX33 using a regular IBM AT-type high-density
> drive. A number of vendors make these.

Actually, you can format RX50 floppies with a special formatting
device, known as a Rainbow.  If you plan to make any use of an RX50
at all, having a Rainbow around to format your floppies (and double as
a terminal) works out pretty well.  I've got a couple rainbows sitting
about that I grabbed just before they hit the garbage.  I've also
seen them sell for less than $100.