Ian, Ketron offers the SD8 with accordion bass and chromatic accordion right hand buttons. I’m pretty sure it has the same specs as the SD5.
I bought my SD1 Plus from Bill Stewart who purchased it from Ketron, had it sent to MasterSound that installed the bass unit. That was some years ago, when the Plus was first introduced, and it was very expensive, I thought ($5,400 back then). I followed Bill’s posts on another forum for years and asked him to let me know if ever thought about selling it. He notified me that he was posting it on ebay and the rest is history. I’ve had the Ajamsonic upgrade installed within the last year.
Ciao,
Jerry