James Perrett wrote:... the outputs on my interface are impedance balanced rather than fully balanced.
I know you know this James, but for the benefit of others whoc might be wondering, 'impedance-balanced' is 'fully balanced' -- as in fully capable of rejecting common-mode interference.
The terms are confusing and misleading for many, so I prefer to say 'asymmetrical' or 'symmetrical' when talking about balanced connections.
The traditional balanced line is engineered to transmit the half the wanted audio on each 'leg' as symmetrical contributions, while impedance-balanced lines transmit all of the wanted signal just on one 'leg'. The main benefit is that this manages to maintain the same signal level whether connected to either balanced or unbalanced destinations.