I am in exactly the same situation.
People understandably enough are saying that one needs a load in order to safely operate a tube amp.
I however wonder whether an audio interface, together with its studio monitors connected to the interface's line out, do in fact act as a load or not.
If they do, then I would not think that a cabinet connected to the amp's line out would be required while the amp is also connected to the interface via the emulated speaker line out connection.
On the other hand I am still choosing which amp to buy. I cannot check first-hand if a signal from the guitar would be fed from the amp o to the interface if a cab is not connected to the standard line out at the same time.
In essence, if the above (ie: guitar to tube amps' line-in; amp's emulated line-out into audio interface's line-in; interface's line-out into studio monitors - with no cab connected into amp's standard line-out) were to be doable, the audio interface would also be acting as an attenuator.