> Ok, I just got home and I figured out for myself that the A-131 is AC

coupled.

So you own probably the first version with CEM3381. The last A-130 with
CEM3381/PA381 were manufactured ~ 2001.

> Is it possible to modify it to be DC

No. The CEM3381/82 VCAs require AC coupling.

Best wishes
Dieter Doepfer