Let me give you a little help here, as well as a benefit to others who may have the same (or similar) question.
Your Shuttle effects send is a pre-master volume post everything else signal. For driving an external amp, it is ment to drive the stage at the input of the power amp before the slave amp's master volume control. MOST bass amps have a serial effects loop that is pre master volume, so the topology is the same at the Shuttle's and when you take the effects send of the Shuttle into the effects return of amp that's driving the speakers. The master volume of the second amp operates just like a normal master volume.
In the case of the Bassman 250, this amp uses a parallel effects loop. This means that there are 2 signal paths (primary or dry, and secondary or wet). There is a balance (blend) control that allows the user to choose between the two signal paths into the power amp. For this application, the balance control must be fully in the wet (or effects return jack) position for signal to pass properly. The master volume will then function as a typical master volume control. On the Bassman 250, as well as some other amps with a parallel loop, a better input to use is the aux input which is after the effects loop signal path, and using this, all you have to worry about is the master volume control.
Hope this helps.
Product Development Engineer: Mesa Engineering
Continuing Product Support: Genz Benz