Does anybody know the ride quality difference between the aluminum frame and carbon frame on the Bacchette recumbent.

I don't. I tested an aluminum Giro once. I could feel all the bumps but it wasn't particularly bothersome. Possibly due to the thick foam on the Euromesh seat. I've never been on a Carbon Aero.

I've got a steel Giro 26 and a carbon fiber CA2.0.
The CA2.0 is plenty stiff, at least in terms of torsion when pedaling. The choice of tire size does affect the ride quality. I've got a 650C wheelset and a 26" wheelset. The 26" wheels can take a bit larger tire, which handles rough roads better.

To me, the biggest difference is the seat. The Giro has the mesh seat with the thick pad for the bottom. The seat structure isn't very stiff laterally, and the bike seems fairly flexible and squirrely.
By comparison, the carbon seat pan and thin foam pad is mounted solidly to the frame. It feels much stiffer in regards to control and bike handling.
I've had both bikes for over 10 years, and have forgotten a lot of the terminology for the seat types... sorry!

Steve in Peoria

Bacchetta offers three seats: the Recurve, which is a hard seatpan with a cushion, combined with a mesh back; a carbon shell; and the Euromesh which is an aluminum outer frame in the shape of a carbon shell, with fabric stretched around the frame. Different models come with different seats as standard equipment, but you can always choose a different seat.

FIFY...

There may be a difference but it's magnitudes less than the difference between tires.

I second that, Tire are Everything In the bent world.

I ride Chromoly steel and love it. All three of my bents are Chromo btw