The BHA1 has both a single XLR4(f) and a pair of XLR3(f) on the front.
These days, balanced headphones have more or less standardized on XLR4(m) for their plugs. When balanced headphones were still a new thing however, the amps were essentially just bridged monoblocks and each monoblock used a combo TRS/XLR3(f) jack. Thus the dual xlr3(m) was the standard on headphones for a little while before xlr4(m) caught on.